gh-151728: Clear the typing caches at interpreter shutdown (GH-154858)

gh-151728: Clear the typing caches at interpreter shutdown (GH-154858)

The typing module caches every subscripted type. When an extension module
leaks a reference to typing, these caches also keep types owned by other
(correct) extension modules alive past interpreter shutdown, where nothing
can free them anymore. The cache_clear callables are already collected in
typing._cleanups, so registering them with atexit is enough to avoid this.