[issue39573] Make PyObject an opaque structure in the limited C API

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

> "static inline" functions are not opaque

I'm well aware of that :-) But once the CPython code base will stop accessing directly PyObject fields directly, it would become possible to experiment changing PyObject layout, at least testing it in CPython.

First changes are just to prepare the code base to experiment the real change. But as Serhiy pointed out, the second part will have an impact on performance and so should be carefully benchmarked to balance advantages and drawbacks, even if it's only done in the limited C API.

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