Type hint of DataFrame(...dtype...) excludes numpy scalar types such as np.int8
#### Code Sample

```python
import pandas as pd
import numpy as np

d = {'col1': [1, 2], 'col2': [3, 4]}
df = pd.DataFrame(data=d, dtype=np.int8)
```
#### Problem description

The code sample above was taken straight from the API reference:
https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.html

It results in a 'Type checker' inspection warning from PyCharm because `np.int8` is not an instance of the expected type `Union[str, np.dtype, "ExtensionDtype"]`. Indeed, `isinstance(np.int8, np.dtype)` is false and the numpy documentation requires a conversion to get a data type object, such as `np.dtype(np.int8)`.

#### Suggested solution

Change the type hint in DataFrame.__init__ to
```python
Union[str, np.dtype, np.generic, "ExtensionDtype"]
```

#### Less desirable solution

Change the API reference to use `np.dtype(np.int8)` instead. This would be a shame, because the current version is more elegant and works perfectly at runtime.

#### Expected Output

The code should not generate type checker warnings.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-28-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8
pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 42.0.2
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.11.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
