bpo-35081: Move _PyGC_FINALIZED() back to C API (GH-10626)

bpo-35081: Move _PyGC_FINALIZED() back to C API (GH-10626)

Partially revert commit 1a6be91e6fd65ce9cb88cbbbb193db7e92ec6076,
move back PyGC API from the internal API to the C API:

* _PyGCHead_NEXT(g), _PyGCHead_SET_NEXT(g, p)
* _PyGCHead_PREV(g), _PyGCHead_SET_PREV(g, p)
* _PyGCHead_FINALIZED(g), _PyGCHead_SET_FINALIZED(g)
* _PyGC_FINALIZED(o), _PyGC_SET_FINALIZED(o)
* _PyGC_PREV_MASK_FINALIZED
* _PyGC_PREV_MASK_COLLECTING
* _PyGC_PREV_SHIFT
* _PyGC_PREV_MASK

_PyObject_GC_TRACK(o) and _PyObject_GC_UNTRACK(o) remain in the
internal API.