bpo-35081: Move _PyObject_GC_TRACK() to pycore_object.h

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• Create Include/internal/pycore_object.h
• Move _PyObject_GC_TRACK() and _PyObject_GC_UNTRACK() macros to
  pycore_object.h
• Remove _PyGC_generation0: replaced with _PyRuntime.gc.generation0
• Replace _PyObject_GC_UNTRACK() with PyObject_GC_UnTrack()
  in _queuemodule.c. The file is compiled without Py_BUILD_CORE.
• Python.h now automatically includes pycore_object.h

https://bugs.python.org/issue35081

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Python.h now automatically includes pycore_object.h

Without that, at least 33 C files has to be modified to add include pycore_object.h: see my first attempt, PR #10272.

I prefer to have smaller PR, easier to review. The automatic include can be removed later.

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Note for myself: PR #10275 has to be updated if this PR is merged before.

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PR #10507 has been merged: I rewrote and rebased this change on top of master.

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I don't understand why compilation fails on AppVeyor:

c:\projects\cpython\python\pytime.c(581): error C2037: left of 'tv_sec' specifies undefined struct/union 'timeval' [C:\projects\cpython\PCbuild\pythoncore.vcxproj] 

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I merged PR #10640 instead.