AssertionError creating array with StringDtype instead of StringDtype(), can we have a friendlier error message? 
The following fails with an `AssertionError` on pandas 1.0.0rc0:

```python
pd.Series(["a", "b"], dtype=pd.StringDtype)
```

I wrote this by mistake — this works:

```python
pd.Series(["a", "b"], dtype=pd.StringDtype())
```

When running the former, I get a traceback with an AssertionError:

```
Traceback (most recent call last):
  File "/media/nas/x21324/miniconda3/envs/py37e/lib/python3.7/site-packages/pandas/core/construction.py", line 534, in _try_cast
    if is_integer_dtype(dtype):
  File "/media/nas/x21324/miniconda3/envs/py37e/lib/python3.7/site-packages/pandas/core/dtypes/common.py", line 827, in is_integer_dtype
    return _is_dtype_type(arr_or_dtype, classes_and_not_datetimelike(np.integer))
  File "/media/nas/x21324/miniconda3/envs/py37e/lib/python3.7/site-packages/pandas/core/dtypes/common.py", line 1732, in _is_dtype_type
    return condition(np.dtype(arr_or_dtype).type)
TypeError: data type not understood

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "mwe18.py", line 2, in <module>
    pd.Series(["a", "b"], dtype=pd.StringDtype)
  File "/media/nas/x21324/miniconda3/envs/py37e/lib/python3.7/site-packages/pandas/core/series.py", line 304, in __init__
    data = sanitize_array(data, index, dtype, copy, raise_cast_failure=True)
  File "/media/nas/x21324/miniconda3/envs/py37e/lib/python3.7/site-packages/pandas/core/construction.py", line 438, in sanitize_array
    subarr = _try_cast(data, dtype, copy, raise_cast_failure)
  File "/media/nas/x21324/miniconda3/envs/py37e/lib/python3.7/site-packages/pandas/core/construction.py", line 562, in _try_cast
    subarr = array_type(arr, dtype=dtype, copy=copy)
  File "/media/nas/x21324/miniconda3/envs/py37e/lib/python3.7/site-packages/pandas/core/arrays/string_.py", line 180, in _from_sequence
    assert dtype == "string"
AssertionError
```

Coming from numpy, I find it unconventional that the type ending in `dtype` is not itself a valid dtype, but rather a constructor for such a dtype.  Therefore, I would consider that a friendlier error message could be in order (or maybe it should even work).

I'm running pandas 1.0.0rc0 on Python 3.7.6.

The full environment:

<details>
# packages in environment at /media/nas/x21324/miniconda3/envs/py37e:
#
# Name                    Version                   Build  Channel
_libgcc_mutex             0.1                 conda_forge    conda-forge
_openmp_mutex             4.5                       0_gnu    conda-forge
affine                    2.3.0                      py_0    conda-forge
aggdraw                   1.3.11           py37hc56bac0_0    conda-forge
alabaster                 0.7.12                     py_0    conda-forge
appdirs                   1.4.3                    pypi_0    pypi
arrow-cpp                 0.15.1           py37had5782a_4    conda-forge
asciitree                 0.3.3                      py_2    conda-forge
attrs                     19.3.0                     py_0    conda-forge
awscli                    1.17.8                   py37_0    conda-forge
babel                     2.8.0                      py_0    conda-forge
backcall                  0.1.0                      py_0    conda-forge
beautifulsoup4            4.8.2                    py37_0    conda-forge
behave                    1.2.6                 py37_1000    conda-forge
bitstring                 3.1.6                    pypi_0    pypi
bleach                    3.1.0                      py_0    conda-forge
blosc                     1.17.0               he1b5a44_1    conda-forge
bokeh                     1.4.0                    py37_0    conda-forge
boost-cpp                 1.70.0               h8e57a91_2    conda-forge
boto3                     1.11.8                     py_0    conda-forge
botocore                  1.14.8                     py_0    conda-forge
brotli                    1.0.7             he1b5a44_1000    conda-forge
bzip2                     1.0.8                h516909a_2    conda-forge
c-ares                    1.15.0            h516909a_1001    conda-forge
ca-certificates           2019.11.28           hecc5488_0    conda-forge
cairo                     1.16.0            hfb77d84_1002    conda-forge
cartopy                   0.17.0          py37hc5b2455_1008    conda-forge
certifi                   2019.11.28               py37_0    conda-forge
cffi                      1.13.2           py37h8022711_0    conda-forge
cfgrib                    0.9.7.3                    py_0    conda-forge
cfitsio                   3.470                hb60a0a2_2    conda-forge
cftime                    1.0.4.2          py37hc1659b7_0    conda-forge
chardet                   3.0.4                 py37_1003    conda-forge
click                     7.0                        py_0    conda-forge
click-plugins             1.1.1                      py_0    conda-forge
cligj                     0.5.0                      py_0    conda-forge
cloudpickle               1.2.2                      py_1    conda-forge
codecov                   2.0.15                     py_1    conda-forge
colorama                  0.4.1                      py_0    conda-forge
colorcet                  2.0.1                      py_0    conda-forge
configobj                 5.0.6                      py_0    conda-forge
coverage                  5.0.3            py37h516909a_0    conda-forge
coveralls                 1.10.0                     py_0    conda-forge
cryptography              2.8              py37h72c5cf5_1    conda-forge
curl                      7.65.3               hf8cf82a_0    conda-forge
cycler                    0.10.0                     py_2    conda-forge
cython                    0.29.14          py37he1b5a44_0    conda-forge
cytoolz                   0.10.1           py37h516909a_0    conda-forge
dask                      2.9.2                      py_0    conda-forge
dask-core                 2.9.2                      py_0    conda-forge
datashader                0.8.0                      py_0    conda-forge
datashape                 0.5.4                      py_1    conda-forge
dbus                      1.13.6               he372182_0    conda-forge
decorator                 4.4.1                      py_0    conda-forge
defusedxml                0.6.0                      py_0    conda-forge
distributed               2.9.3                      py_0    conda-forge
docopt                    0.6.2                      py_1    conda-forge
docutils                  0.15.2                   py37_0    conda-forge
double-conversion         3.1.5                he1b5a44_2    conda-forge
eccodes                   2.14.1               hc4140ea_3    conda-forge
entrypoints               0.3                   py37_1000    conda-forge
expat                     2.2.5             he1b5a44_1004    conda-forge
fasteners                 0.14.1                     py_3    conda-forge
fcitools                  0.1.0.post0.dev5+ga19d0d1          pypi_0    pypi
fiona                     1.8.13           py37h900e953_0    conda-forge
flake8                    3.7.9                    py37_0    conda-forge
fogpy                     1.2.0+dev21.g2b0d49d.get.validation.output.for.fog.mask          pypi_0    pypi
fogtools                  0.0.post0.dev13+g735723f.dirty           dev_0    <develop>
fontconfig                2.13.1            h86ecdb6_1001    conda-forge
freetype                  2.10.0               he983fc9_1    conda-forge
freetype-py               2.1.0.post1                py_0    conda-forge
freexl                    1.0.5             h14c3975_1002    conda-forge
fsspec                    0.6.2                      py_0    conda-forge
g2clib                    1.6.0                hf3f1b0b_9    conda-forge
gdal                      3.0.2            py37hbb6b9fb_3    conda-forge
geopandas                 0.6.2                      py_0    conda-forge
geos                      3.7.2                he1b5a44_2    conda-forge
geotiff                   1.5.1                hbd99317_7    conda-forge
geoviews                  1.6.5                      py_0    conda-forge
geoviews-core             1.6.5                      py_0    conda-forge
gettext                   0.19.8.1          hc5be6a0_1002    conda-forge
gflags                    2.2.2             he1b5a44_1002    conda-forge
giflib                    5.1.7                h516909a_1    conda-forge
glib                      2.58.3          py37h6f030ca_1002    conda-forge
glog                      0.4.0                he1b5a44_1    conda-forge
goesr                     0.5.2                    pypi_0    pypi
grpc-cpp                  1.25.0               h5321d42_1    conda-forge
gst-plugins-base          1.14.5               h0935bb2_0    conda-forge
gstreamer                 1.14.5               h36ae1b5_0    conda-forge
h5netcdf                  0.7.4                      py_0    conda-forge
h5py                      2.10.0          nompi_py37h513d04c_101    conda-forge
hdf4                      4.2.13            hf30be14_1003    conda-forge
hdf5                      1.10.5          nompi_h3c11f04_1104    conda-forge
hdfeos2                   2.20              h64bfcee_1000    conda-forge
hdfeos5                   5.1.16               h8b6279f_5    conda-forge
heapdict                  1.0.1                      py_0    conda-forge
holoviews                 1.12.7                     py_0    conda-forge
icu                       64.2                 he1b5a44_1    conda-forge
idna                      2.8                   py37_1000    conda-forge
imageio                   2.6.1                    py37_0    conda-forge
imagesize                 1.2.0                      py_0    conda-forge
importlib_metadata        1.4.0                    py37_0    conda-forge
ipdb                      0.12.3                     py_0    conda-forge
ipykernel                 5.1.3            py37h5ca1d4c_0    conda-forge
ipython                   7.11.1           py37h5ca1d4c_0    conda-forge
ipython_genutils          0.2.0                      py_1    conda-forge
ipywidgets                7.5.1                      py_0    conda-forge
jasper                    1.900.1           h07fcdf6_1006    conda-forge
jedi                      0.15.2                   py37_0    conda-forge
jinja2                    2.10.3                     py_0    conda-forge
jmespath                  0.9.4                      py_0    conda-forge
jpeg                      9c                h14c3975_1001    conda-forge
json-c                    0.13.1            h14c3975_1001    conda-forge
jsonschema                3.2.0                    py37_0    conda-forge
jupyter                   1.0.0                      py_2    conda-forge
jupyter_client            5.3.4                    py37_1    conda-forge
jupyter_console           6.0.0                      py_0    conda-forge
jupyter_core              4.6.1                    py37_0    conda-forge
kealib                    1.4.10            h58c409b_1005    conda-forge
kiwisolver                1.1.0            py37hc9558a2_0    conda-forge
krb5                      1.16.4               h2fd8d38_0    conda-forge
ld_impl_linux-64          2.33.1               h53a641e_7    conda-forge
libaec                    1.0.4                hf484d3e_0    conda-forge
libblas                   3.8.0               14_openblas    conda-forge
libcblas                  3.8.0               14_openblas    conda-forge
libclang                  9.0.1           default_hde54327_0    conda-forge
libcurl                   7.65.3               hda55be3_0    conda-forge
libdap4                   3.20.4               hd3bb157_0    conda-forge
libedit                   3.1.20170329      hf8c457e_1001    conda-forge
libevent                  2.1.10               h72c5cf5_0    conda-forge
libffi                    3.2.1             he1b5a44_1006    conda-forge
libgcc-ng                 9.2.0                h24d8f2e_2    conda-forge
libgdal                   3.0.2                h5534617_3    conda-forge
libgfortran-ng            7.3.0                hdf63c60_4    conda-forge
libgomp                   9.2.0                h24d8f2e_2    conda-forge
libiconv                  1.15              h516909a_1005    conda-forge
libkml                    1.3.0             h4fcabce_1010    conda-forge
liblapack                 3.8.0               14_openblas    conda-forge
libllvm8                  8.0.1                hc9558a2_0    conda-forge
libllvm9                  9.0.1                hc9558a2_0    conda-forge
libnetcdf                 4.7.1           nompi_h94020b1_102    conda-forge
libopenblas               0.3.7                h5ec1e0e_6    conda-forge
libpng                    1.6.37               hed695b0_0    conda-forge
libpq                     11.5                 hd9ab2ff_2    conda-forge
libprotobuf               3.10.1               h8b12597_0    conda-forge
libsodium                 1.0.17               h516909a_0    conda-forge
libspatialindex           1.9.3                he1b5a44_1    conda-forge
libspatialite             4.3.0a            h4f6d029_1032    conda-forge
libssh2                   1.8.2                h22169c7_2    conda-forge
libstdcxx-ng              9.2.0                hdf63c60_2    conda-forge
libtiff                   0.4.2                    pypi_0    pypi
libuuid                   2.32.1            h14c3975_1000    conda-forge
libwebp                   1.0.2                hf4e8a37_4    conda-forge
libxcb                    1.13              h14c3975_1002    conda-forge
libxkbcommon              0.9.1                hebb1f50_0    conda-forge
libxml2                   2.9.10               hee79883_0    conda-forge
libxslt                   1.1.33               h31b3aaa_0    conda-forge
line_profiler             3.0.2            py37hc9558a2_0    conda-forge
llvmlite                  0.31.0           py37h8b12597_0    conda-forge
locket                    0.2.0                      py_2    conda-forge
lxml                      4.4.2            py37h7ec2d77_0    conda-forge
lz4-c                     1.8.3             he1b5a44_1001    conda-forge
lzo                       2.10              h14c3975_1000    conda-forge
markupsafe                1.1.1            py37h516909a_0    conda-forge
matplotlib                3.1.2                    py37_1    conda-forge
matplotlib-base           3.1.2            py37h250f245_1    conda-forge
mccabe                    0.6.1                      py_1    conda-forge
mechanicalsoup            0.12.0                     py_0    conda-forge
mistune                   0.8.4           py37h516909a_1000    conda-forge
mock                      3.0.5                    py37_0    conda-forge
monotonic                 1.5                        py_0    conda-forge
more-itertools            8.1.0                      py_0    conda-forge
msgpack-python            0.6.2            py37hc9558a2_0    conda-forge
multipledispatch          0.6.0                      py_0    conda-forge
munch                     2.5.0                      py_0    conda-forge
nbconvert                 5.6.1                    py37_0    conda-forge
nbformat                  5.0.4                      py_0    conda-forge
ncurses                   6.1               hf484d3e_1002    conda-forge
netcdf4                   1.5.3           nompi_py37hfee4bf2_101    conda-forge
networkx                  2.4                        py_0    conda-forge
notebook                  6.0.3                    py37_0    conda-forge
nspr                      4.24                 he1b5a44_0    conda-forge
nss                       3.47                 he751ad9_0    conda-forge
numba                     0.47.0           py37hb3f55d8_0    conda-forge
numcodecs                 0.6.4            py37he1b5a44_0    conda-forge
numexpr                   2.7.1            py37hb3f55d8_0    conda-forge
numpy                     1.17.5           py37h95a1406_0    conda-forge
olefile                   0.46                       py_0    conda-forge
opencv-python             4.1.0.25                 pypi_0    pypi
openjpeg                  2.3.1                h981e76c_3    conda-forge
openssl                   1.1.1d               h516909a_0    conda-forge
owslib                    0.19.0                     py_2    conda-forge
packaging                 20.0                       py_0    conda-forge
pandas                    1.0.0rc0                 pypi_0    pypi
pandoc                    2.9.1.1                       0    conda-forge
pandocfilters             1.4.2                      py_1    conda-forge
param                     1.9.2                      py_0    conda-forge
parquet-cpp               1.5.1                         2    conda-forge
parse                     1.14.0                     py_0    conda-forge
parse_type                0.5.2                    py37_0    conda-forge
parso                     0.5.2                      py_0    conda-forge
partd                     1.1.0                      py_0    conda-forge
pcre                      8.43                 he1b5a44_0    conda-forge
pexpect                   4.8.0                    py37_0    conda-forge
pickleshare               0.7.5                 py37_1000    conda-forge
pillow                    7.0.0            py37hefe7db6_0    conda-forge
pip                       20.0.1                   py37_0    conda-forge
pixman                    0.38.0            h516909a_1003    conda-forge
pluggy                    0.13.0                   py37_0    conda-forge
poppler                   0.67.0               h14e79db_8    conda-forge
poppler-data              0.4.9                         1    conda-forge
postgresql                11.5                 hc63931a_2    conda-forge
progressbar2              3.47.0                     py_0    conda-forge
proj                      6.2.1                hc80f0dc_0    conda-forge
prometheus_client         0.7.1                      py_0    conda-forge
prompt_toolkit            2.0.10                     py_0    conda-forge
psutil                    5.6.7            py37h516909a_0    conda-forge
pthread-stubs             0.4               h14c3975_1001    conda-forge
ptyprocess                0.6.0                   py_1001    conda-forge
py                        1.8.1                      py_0    conda-forge
pyarrow                   0.15.1           py37h8b68381_1    conda-forge
pyasn1                    0.4.8                      py_0    conda-forge
pycoast                   1.3.2                      py_0    conda-forge
pycodestyle               2.5.0                      py_0    conda-forge
pycparser                 2.19                     py37_1    conda-forge
pyct                      0.4.6                      py_0    conda-forge
pyct-core                 0.4.6                      py_0    conda-forge
pydap                     3.2.2                 py37_1000    conda-forge
pydecorate                0.2.1                      py_0    conda-forge
pyepsg                    0.4.0                      py_0    conda-forge
pyflakes                  2.1.1                      py_0    conda-forge
pygments                  2.5.2                      py_0    conda-forge
pygrib                    2.0.4            py37h55c65ac_0    conda-forge
pyhdf                     0.10.2           py37h3a4e923_0    conda-forge
pykdtree                  1.3.1           py37hc1659b7_1002    conda-forge
pynio                     1.5.5            py37h2e5bc1a_7    conda-forge
pyopengl                  3.1.5                      py_0    conda-forge
pyopenssl                 19.1.0                   py37_0    conda-forge
pyorbital                 1.5.0                    pypi_0    pypi
pyparsing                 2.4.6                      py_0    conda-forge
pyproj                    2.4.2.post1      py37h12732c1_0    conda-forge
pyqt                      5.12.3           py37hcca6a23_1    conda-forge
pyqt5                     5.13.1                   pypi_0    pypi
pyqt5-sip                 4.19.18                  pypi_0    pypi
pyqtgraph                 0.11.0rc0                  py_0    conda-forge
pyqtwebengine             5.12.1                   pypi_0    pypi
pyresample                1.14.0           py37hb3f55d8_2    conda-forge
pyrsistent                0.15.7           py37h516909a_0    conda-forge
pyscaffold                3.2.2                      py_0    conda-forge
pyshp                     2.1.0                      py_0    conda-forge
pysocks                   1.7.1                    py37_0    conda-forge
pyspectral                0.8.9                    pypi_0    pypi
pytables                  3.6.1            py37h9f153d1_1    conda-forge
pytest                    5.3.4                    py37_0    conda-forge
pytest-cov                2.8.1                      py_0    conda-forge
pytest-mock               2.0.0                    py37_0    conda-forge
pytest-runner             5.2                        py_0    conda-forge
python                    3.7.6                h357f687_2    conda-forge
python-dateutil           2.8.1                      py_0    conda-forge
python-geotiepoints       1.1.8                    pypi_0    pypi
python-utils              2.3.0                      py_1    conda-forge
pytz                      2019.3                     py_0    conda-forge
pyviz_comms               0.7.3                      py_0    conda-forge
pywavelets                1.1.1            py37hc1659b7_0    conda-forge
pyyaml                    5.2              py37h516909a_0    conda-forge
pyzmq                     18.1.1           py37h1768529_0    conda-forge
qt                        5.12.5               hd8c4c69_1    conda-forge
qtconsole                 4.6.0                      py_0    conda-forge
quamash                   0.6.1                    pypi_0    pypi
rasterio                  1.1.1            py37h900e953_0    conda-forge
re2                       2020.01.01           he1b5a44_0    conda-forge
readline                  8.0                  hf8c457e_0    conda-forge
requests                  2.22.0                   py37_1    conda-forge
rsa                       3.4.2                      py_1    conda-forge
rtree                     0.9.3            py37h7b0cdae_0    conda-forge
s3fs                      0.4.0                      py_0    conda-forge
s3transfer                0.3.2                    py37_0    conda-forge
satpy                     0.19.2.dev31+gf537a14d          pypi_0    pypi
scikit-image              0.16.2           py37hb3f55d8_0    conda-forge
scipy                     1.4.1            py37h921218d_0    conda-forge
send2trash                1.5.0                      py_0    conda-forge
setuptools                45.1.0                   py37_0    conda-forge
shapely                   1.6.4           py37hec07ddf_1006    conda-forge
six                       1.14.0                   py37_0    conda-forge
snakeviz                  2.0.1                      py_0    conda-forge
snappy                    1.1.7             he1b5a44_1003    conda-forge
snowballstemmer           2.0.0                      py_0    conda-forge
snuggs                    1.4.7                      py_0    conda-forge
sortedcontainers          2.1.0                      py_0    conda-forge
soupsieve                 1.9.4                    py37_0    conda-forge
sphinx                    2.3.1                      py_0    conda-forge
sphinxcontrib-applehelp   1.0.1                      py_0    conda-forge
sphinxcontrib-devhelp     1.0.1                      py_0    conda-forge
sphinxcontrib-htmlhelp    1.0.2                      py_0    conda-forge
sphinxcontrib-jsmath      1.0.1                      py_0    conda-forge
sphinxcontrib-qthelp      1.0.2                      py_0    conda-forge
sphinxcontrib-serializinghtml 1.1.3                      py_0    conda-forge
sqlalchemy                1.3.13           py37h516909a_0    conda-forge
sqlite                    3.30.1               hcee41ef_0    conda-forge
tbb                       2018.0.5             h2d50403_0    conda-forge
tblib                     1.6.0                      py_0    conda-forge
terminado                 0.8.3                    py37_0    conda-forge
testpath                  0.4.4                      py_0    conda-forge
thrift-cpp                0.12.0            hf3afdfd_1004    conda-forge
tiledb                    1.7.0                hcde45ca_2    conda-forge
tk                        8.6.10               hed695b0_0    conda-forge
toolz                     0.10.0                     py_0    conda-forge
tornado                   6.0.3            py37h516909a_0    conda-forge
traitlets                 4.3.3                    py37_0    conda-forge
trollbufr                 0.11.0                   pypi_0    pypi
trollimage                1.9.0                    pypi_0    pypi
trollsift                 0.3.2                    pypi_0    pypi
tzcode                    2019a             h516909a_1002    conda-forge
uriparser                 0.9.3                he1b5a44_1    conda-forge
urllib3                   1.25.7                   py37_0    conda-forge
uwsift                    1.0.6                    pypi_0    pypi
vispy                     0.6.4            py37hf5ce8a7_0    conda-forge
wcwidth                   0.1.8                      py_0    conda-forge
webencodings              0.5.1                      py_1    conda-forge
webob                     1.8.6                      py_0    conda-forge
wheel                     0.33.6                   py37_0    conda-forge
widgetsnbextension        3.5.1                    py37_0    conda-forge
xarray                    0.14.1                     py_0    conda-forge
xerces-c                  3.2.2             h8412b87_1004    conda-forge
xorg-kbproto              1.0.7             h14c3975_1002    conda-forge
xorg-libice               1.0.10               h516909a_0    conda-forge
xorg-libsm                1.2.3             h84519dc_1000    conda-forge
xorg-libx11               1.6.9                h516909a_0    conda-forge
xorg-libxau               1.0.9                h14c3975_0    conda-forge
xorg-libxdmcp             1.1.3                h516909a_0    conda-forge
xorg-libxext              1.3.4                h516909a_0    conda-forge
xorg-libxrender           0.9.10            h516909a_1002    conda-forge
xorg-renderproto          0.11.1            h14c3975_1002    conda-forge
xorg-xextproto            7.3.0             h14c3975_1002    conda-forge
xorg-xproto               7.0.31            h14c3975_1007    conda-forge
xz                        5.2.4             h14c3975_1001    conda-forge
yaml                      0.2.2                h516909a_1    conda-forge
zarr                      2.4.0                      py_0    conda-forge
zeromq                    4.3.2                he1b5a44_2    conda-forge
zict                      1.0.0                      py_0    conda-forge
zipp                      2.0.0                      py_2    conda-forge
zlib                      1.2.11            h516909a_1006    conda-forge
zstd                      1.4.4                h3b9ef0a_1    conda-forge
</details>




```python
# Your code here

```