bpo-46070: _PyGC_Fini() untracks objects (GH-30577) (GH-30580)

Py_EndInterpreter() now explicitly untracks all objects currently
tracked by the GC. Previously, if an object was used later by another
interpreter, calling PyObject_GC_UnTrack() on the object crashed if
the previous or the next object of the PyGC_Head structure became a
dangling pointer.

(cherry picked from commit 1a4d1c1c9b08e75e88aeac90901920938f649832)