ENH: Return type of pandas.DataFrame.dropna is incorrectly None
- [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.

---

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

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

example = pd.DataFrame({'a': [1, 2, 3], 'b': [4, np.nan, 6]})
example.dropna()
```


#### Problem description

Currently, the return type of `pandas.DataFrame.dropna` is missing. The documentation correctly states that the return type should be `Optional[DataFrame]`, but the function definition lacks a return type, so type checkers (in my case pyright) incorrectly throw the type as `None`.

The issue occurs in the source code here:
https://github.com/pandas-dev/pandas/blob/3e89b4c4b1580aa890023fc550774e63d499da25/pandas/core/frame.py#L5041

It should be typed as seen in `pandas.DataFrame.drop_duplicates`, seen below:
https://github.com/pandas-dev/pandas/blob/3e89b4c4b1580aa890023fc550774e63d499da25/pandas/core/frame.py#L5183-L5189

More broadly, it seems that the `dropna` function is missing typing for its arguments as well.

#### Expected Output

The return type of `pandas.DataFrame.dropna` should be `Optional[DataFrame]`

I don't think this should be too hard to fix. I'll work in it tonight and submit a PR in the next day or two.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.9.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.8.0-33-generic
Version          : #36-Ubuntu SMP Wed Dec 9 09:14:40 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.2.0
numpy            : 1.19.4
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.2.2
setuptools       : 51.0.0
Cython           : None
pytest           : 6.2.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None


</details>
