gh-129185: Fix PyTraceMalloc_Untrack() at Python exit (#129191)

Support calling PyTraceMalloc_Track() and PyTraceMalloc_Untrack()
during late Python finalization.

* Call _PyTraceMalloc_Fini() later in Python finalization.
* Test also PyTraceMalloc_Untrack() without the GIL
* PyTraceMalloc_Untrack() now gets the GIL.
* Test also PyTraceMalloc_Untrack() in test_tracemalloc_track_race().