gh-89653: PEP 670: Convert PyCFunction macros to functions

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Convert the following macros to static inline functions:

• PyCFunction_GET_CLASS()
• PyCFunction_GET_FLAGS()
• PyCFunction_GET_FUNCTION()
• PyCFunction_GET_SELF()

Limited C API version 3.11 no longer casts arguments.

Export the PyCMethod_GetClass() function.

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Export the PyCMethod_GetClass() function.

It seems like it was missed when PEP 573 was implemented in Python 3.9: cc @encukou @ericsnowcurrently @ncoghlan

[encukou]

Hm, PyCMethod_GetClass was left in by mistake from the initial implementation. I'd rather remove it than export it.
Per PEP-573, the API for this is PyCFunction_GET_CLASS. No PyCMethod_* functions were planned, all PyCFunction_* should work on PyCMethod as well.
(PyCMethod_Check* were added later, in 4c9ea09)

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Hm, PyCMethod_GetClass was left in by mistake from the initial implementation. I'd rather remove it than export it.

Ah. In that change, I leave it unchanged in my PR which is unrelated.

Per PEP-573, the API for this is PyCFunction_GET_CLASS.

The problem with macros and static inline functions is that they are not usable in some programming languages, and they usually leak too many implementation details to be part of the limited C API. Since each PyCFunction macro or static inline function is paired with a regular function, to me, it looked consistent to expose PyCMethod_GetClass(). But it's up to you ;-)

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I'm thinking about deprecating macros and static inline functions, and maybe later make them an alias of their related function. I already did for PyThreadState_GET() which now just calls PyThreadState_Get(). IMO these macros and static inline functions only exist for best performance for CPython internals, but the benefits of using them outside CPython is not significant (in terms of performance). They make sense in the internal C API, not for the regular public C API. But well, that's a whole different topic, I should run benchmarks and write a PEP ;-)

[encukou]

The problem with macros and static inline functions is that they are not usable in some programming languages, and they usually leak too many implementation details to be part of the limited C API. Since each PyCFunction macro or static inline function is paired with a regular function, to me, it looked consistent to expose PyCMethod_GetClass(). But it's up to you ;-)

Oh, I'm not insisting on this being a macro. But the function should be PyCFunction_GetClass rather than PyCMethod_*.
(The fact that the struct is different – that is, space for the class pointer isn't allocated – should be an implementation detail.)