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)