[3.13] gh-119560: Drop an Invalid Assert in PyState_FindModule() (gh-119561) (gh-119632)

The assertion was added in gh-118532 but was based on the invalid assumption that PyState_FindModule() would only be called with an already-initialized module def.  I've added a test to make sure we don't make that assumption again.

(cherry picked from commit 0c5ebe13e9937c446e9947c44f2570737ecca135)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>