[3.9] bpo-37788: Fix reference leak when Thread is never joined (GH-26103) (GH-26142)

[3.9] bpo-37788: Fix reference leak when Thread is never joined (GH-26103) (GH-26142)

When a Thread is not joined after it has stopped, its lock may remain in the _shutdown_locks set until interpreter shutdown.  If many threads are created this way, the _shutdown_locks set could therefore grow endlessly.  To avoid such a situation, purge expired locks each time a new one is added or removed..
(cherry picked from commit c10c2ec7a0e06975e8010c56c9c3270f8ea322ec)

Co-authored-by: Antoine Pitrou <antoine@python.org>
Automerge-Triggered-By: GH:pitrou