TYP: annotation of __init__ return type (PEP 484) (pandas/plotting)
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

According to [documentation](https://pandas.pydata.org/docs/dev/development/contributing_codebase.html#type-hints) pandas uses [PEP484](https://www.python.org/dev/peps/pep-0484/) for type hints.
PEP484 states that `__init__` function should have `-> None` return type annotation.
In this PR I added return type annotation to `__init__` functions in `pandas/plotting` module.
Thanks!

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=75016&view=logs&j=b1891a80-bdb0-5193-5f59-ce68a0874df0&t=0a58a9e9-0673-539c-5bde-d78a5a3b655e

these might be legit thanks @nafarya FYI we are going to do these in a few PRs that cover most of the codebase