Drop support for Python 3.9

[niklasf]

• int.bit_count
• typing.TypeAlias instead of aliases configured in doc/conf.py
• typing.Union syntax

[bswck]

TypeAlias can already be used from typing_extensions (with no need of actually having typing_extensions installed, mypy looks up the stub in the typeshed and sees it as a part of the standard library).

[niklasf]

@bswck Interesting. Won't import typing_extensions fail at runtime if it's not present, or is there a way to avoid that?

[bswck]

@bswck Interesting. Won't import typing_extensions fail at runtime if it's not present, or is there a way to avoid that?

Won't fail if we don't perform the import at runtime at all, i.e. in the if typing.TYPE_CHECKING: block.
Then we can enable PEP 563 behavior to not evaluate type annotations at runtime.

[niklasf]

Ah, cool. Done.