Start using `|` instead of `Union` or `Optional` when typing

[Illviljan]

Is your feature request related to a problem? Please describe.
To make it easier reading the typing it is now possible to use | instead of Union or Optional.

Here's an example how it looks like in pandas:
https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/_core.py#L116-L134

Describe the solution you'd like
Replace for example:

• Union[str, int] with str | int
• Optional[str] with None | str

This would likely require adding from __future__ import annotations at the top of the file.

References
https://www.python.org/dev/peps/pep-0604/

[max-sixty]

Agree! If we add the heading, pyupgrade will do a full find-replace...