bpo-40170: Remove PyIndex_Check() macro

[vstinner]

Always declare PyIndex_Check() as an opaque function to hide
implementation details: remove PyIndex_Check() macro. The macro
accessed directly the PyTypeObject.tp_as_number member.

https://bugs.python.org/issue40170

[vstinner]

I already modified C files of Python and Objects subdirectories to use a new fast inlined _PyIndex_Check() function in commit a15e260 (GH-19426).

Only the following modules will be impacted by this change:

• Modules/_ctypes/_ctypes.c
• Modules/_elementtree.c
• Modules/_functoolsmodule.c
• Modules/_sre.c
• Modules/_struct.c
• Modules/_testbuffer.c
• Modules/_xxsubinterpretersmodule.c
• Modules/arraymodule.c
• Modules/mmapmodule.c
• Modules/posixmodule.c

These modules will now call a function rather than inlining the code.