[3.13] gh-122191: Fix test_warnings failure if run with -Werror (GH-122222) (GH-122256)

__spec__.loader is now required in the module globals (see gh-86298).
(cherry picked from commit 9b4fe9b718f27352ba0c1cf1184f5b90d77d7df4)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>