Add Py_GenericAlias

[elprans]

Useful for extensions implementing generic container types, like
immutables (see MagicStack/immutables#89).

Co-Authored-By: ToBeReplaced ToBeReplaced@gmail.com

[vstinner]

I'm not sure if this implementation fits well with pythoncapi-compat. Usually, I only provided a fallback implementation when they was a way to implement a feature on old Python. For example, PyFrame_GetBack(frame) really returns the previous frame. And PyThreadState_EnterTracing() and PyThreadState_LeaveTracing() really work as the function works on Python 3.11.

Here, it's not a type alias, it just returns the same input type, unmodified, and the second argument is just ignored. I don't see the value of adding this function to pythoncapi-compat. Why not puting an #ifdef in immutables to have a different code path on Python 3.8 and older?

In shorter, I prefer that the developer of a C extension thinks how they have to deal with Python 3.8 and older.

Sorry, I didn't put any guidelines on what belongs or not to pythoncapi-compat.

[elprans]

I'm not sure if this implementation fits well with pythoncapi-compat

Yeah, that makes sense, thanks for confirming. Easy to handle in the extension directly.