bpo-40241: Add pycore_gc.h header file (GH-19494)

Move the PyGC_Head structure and the following private macros to the
internal C API:

* _PyGCHead_FINALIZED()
* _PyGCHead_NEXT()
* _PyGCHead_PREV()
* _PyGCHead_SET_FINALIZED()
* _PyGCHead_SET_NEXT()
* _PyGCHead_SET_PREV()
* _PyGC_FINALIZED()
* _PyGC_PREV_MASK
* _PyGC_PREV_MASK_COLLECTING
* _PyGC_PREV_MASK_FINALIZED
* _PyGC_PREV_SHIFT
* _PyGC_SET_FINALIZED()
* _PyObject_GC_IS_TRACKED()
* _PyObject_GC_MAY_BE_TRACKED()
* _Py_AS_GC(o)

Keep the private _PyGC_FINALIZED() macro in the public C API for
backward compatibility with Python 3.8: make it an alias to the new
PyObject_GC_IsFinalized() function.

Move the SIZEOF_PYGC_HEAD constant from _testcapi module to
_testinternalcapi module.