CONTRIBUTING: Use PEP 604 syntax

[srittau]

Built-in generics not possible in type aliases

[hoefling]

@srittau it's also not possible to use PEP 604 syntax in class bases, e.g.

class c_char_p(_PointerLike, _SimpleCData[bytes | None]): ...

instead of

typeshed/stdlib/ctypes/__init__.pyi

Line 218 in a6ce6fd

class c_char_p(_PointerLike, _SimpleCData[Optional[bytes]]):

[srittau]

@srittau it's also not possible to use PEP 604 syntax in class bases, e.g.

Good catch, updated.

[Akuli]

I didn't do this in #5879, because I thought it would be easier to say "Use the new syntax", and when needed, say "mypy is still buggy, use the old syntax here" when reviewing.

[hoefling]

FWIW, both issues with PEP 604 syntax are resolved on mypy's master, so the 0.920 / 0.1000 /whatever should obsolete the warnings.