[3.13] gh-117657: Fix data race in `dict_dict_merge` (gh-129755) (gh-129808)

Found while running `test_load_attr_module` from `test_opcache` under TSan.
(cherry picked from commit 34379d0a593e5a76e2f754cdd7fccb79f25a4613)

Co-authored-by: Sam Gross <colesbury@gmail.com>