[3.15] gh-151722: Defer GC tracking in frozendict.copy() (GH-152230) (#152271)

[3.15] gh-151722: Defer GC tracking in frozendict.copy() (GH-152230) (#152271)

gh-151722: Defer GC tracking in frozendict.copy() (GH-152230)

Fix _PyDict_Or() and frozendict.copy(): only track the frozendict by
the GC once the dictionary is fully initialized.

Functions modifying frozendict now ensures that the object is not
tracked by the GC (in debug mode).

* can_modify_dict() checks that _PyObject_GC_IS_TRACKED() is false
  for frozendicts.
* dict_merge_api() makes sure that the dictionary is tracked by the
  GC.
(cherry picked from commit 05679f3576ed3298c9b0f86e8a3462f0b92dff8f)

Co-authored-by: Victor Stinner <vstinner@python.org>