Support enum.member for python3.11+

[sobolevn]

There are no tests for @enum.member used as a decorator, because I can only decorate classes and functions, which are not supported right now: https://mypy-play.net/?mypy=latest&python=3.12&gist=449ee8c12eba9f807cfc7832f1ea2c49

import enum

class A(enum.Enum):
    class x: ...

reveal_type(A.x)  # Revealed type is "def () -> __main__.A.x"

This issue is separate and rather complex, so I would prefer to solve it independently.

Refs #17376

[AlexWaygood]

Nice!

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Thanks, looks good to me!