BUG: Casting string series to category raises TypeError in nightlies
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import pandas.testing as pdt

y = pd.Series(["A"], dtype="object").astype("category")
x = pd.Series(["A"], dtype="string").astype("category")

pdt.assert_series_equal(x,y)
```
`pandas==1.2.3`: runs through, doesn't raise

`pandas==1.3.0.dev0+1005.g93c52e4e1d` (from the nightly wheels): 
```python-traceback
Traceback (most recent call last):
  File "/home/simon/Documents/QuantCo/quantcore.thek/test.py", line 5, in <module>
    x = pd.Series(["A"], dtype="string").astype("category")
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/pandas/core/generic.py", line 5944, in astype
    new_data = self._mgr.astype(dtype=dtype, copy=copy, errors=errors)
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/pandas/core/internals/managers.py", line 659, in astype
    return self.apply("astype", dtype=dtype, copy=copy, errors=errors)
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/pandas/core/internals/managers.py", line 524, in apply
    applied = getattr(b, f)(**kwargs)
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/pandas/core/internals/blocks.py", line 658, in astype
    new_values = astype_array_safe(values, dtype, copy=copy, errors=errors)
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/pandas/core/dtypes/cast.py", line 1352, in astype_array_safe
    new_values = astype_array(values, dtype, copy=copy)
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/pandas/core/dtypes/cast.py", line 1297, in astype_array
    values = values.astype(dtype, copy=copy)
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/pandas/core/arrays/string_.py", line 328, in astype
    elif np.issubdtype(dtype, np.floating):
  File "/home/simon/miniconda3/envs/quantcore.thek/lib/python3.9/site-packages/numpy/core/numerictypes.py", line 419, in issubdtype
    arg1 = dtype(arg1).type
TypeError: Cannot interpret 'CategoricalDtype(categories=None, ordered=None)' as a data type
```

#### Problem description

Seems like a regression. Converting the `string` series to `object` first works without raising:
```python
x = pd.Series(["A"], dtype="string").astype("object").astype("category")
```

#### Expected Output
`nightly` should behave like `pandas=1.2.3`, running through and producing two equal series.
#### Output of ``pd.show_versions()``

<details>



INSTALLED VERSIONS
------------------
commit           : 93c52e4e1d172512b32e0e76e9b09c0cf76daf21
python           : 3.9.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.8.0-44-generic
Version          : #50-Ubuntu SMP Tue Feb 9 06:29:41 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.0.dev0+1005.g93c52e4e1d
numpy            : 1.21.0.dev0+934.g6222e283f
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 49.6.0.post20210108
Cython           : None
pytest           : 6.2.2
hypothesis       : None
sphinx           : 3.5.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.21.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 0.8.7
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 3.1.0.dev317
pyxlsb           : None
s3fs             : None
scipy            : 1.6.0
sqlalchemy       : None
tables           : None
tabulate         : 0.8.9
xarray           : None
xlrd             : None
xlwt             : None
numba            : None


</details>
