[Enum] `__text_signature__` of `EnumType.__call__` and derivatives

[ethanfurman]

EnumType.__call__ is a dual-purpose method:

• create a new enum class (functional API, and only valid if the parent enum has no members)
• look up an existing member (only valid if enum has members)

Enhancement: Have the appropriate __text_signature__ set for each enum/flag.

Linked PRs

• gh-100039: enhance __signature__ to work with str and callables #100168

[ethanfurman]

Okay, __text_signature__ is not settable on a class (so, for example, not on enum.Enum). It would take some changes in the C code and possibly to internal data structures to make that work.

So instead I have a patch that augments inspect to be able to properly interpret __signature__ when it is a classmethod that returns a string.