[3.14] GH-91636: Clear weakrefs created by finalizers. (GH-136401) (#141993)

GH-91636: Clear weakrefs created by finalizers. (GH-136401)

Weakrefs to unreachable garbage that are created during running of
finalizers need to be cleared.  This avoids exposing objects that
have `tp_clear` called on them to Python-level code.
(cherry picked from commit b6b99bf7f1edab77c485faf4e23da868f3a7b68c)

Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>