[3.8] bpo-36402: Fix threading._shutdown() race condition (GH-13948) (GH-14050)

[3.8] bpo-36402: Fix threading._shutdown() race condition (GH-13948) (GH-14050)

* bpo-36402: Fix threading._shutdown() race condition (GH-13948)

Fix a race condition at Python shutdown when waiting for threads.
Wait until the Python thread state of all non-daemon threads get
deleted (join all non-daemon threads), rather than just wait until
Python threads complete.

* Add threading._shutdown_locks: set of Thread._tstate_lock locks
  of non-daemon threads used by _shutdown() to wait until all Python
  thread states get deleted. See Thread._set_tstate_lock().
* Add also threading._shutdown_locks_lock to protect access to
  threading._shutdown_locks.
* Add test_finalization_shutdown() test.

(cherry picked from commit 468e5fec8a2f534f1685d59da3ca4fad425c38dd)

* bpo-36402: Fix threading.Thread._stop() (GH-14047)

Remove the _tstate_lock from _shutdown_locks, don't remove None.

(cherry picked from commit 6f75c873752a16a7ad8f35855b1e29f59d048e84)