gh-117657: Fix data race in dict_dict_merge (gh-129755)

Found while running `test_load_attr_module` from `test_opcache` under TSan.

Author: colesbury

Objects/dictobject.c

@@ -3793,7 +3793,7 @@ dict_dict_merge(PyInterpreterState *interp, PyDictObject *mp, PyDictObject *othe
 
             ensure_shared_on_resize(mp);
             dictkeys_decref(interp, mp->ma_keys, IS_DICT_SHARED(mp));
-            mp->ma_keys = keys;
+            set_keys(mp, keys);
             STORE_USED(mp, other->ma_used);
             ASSERT_CONSISTENT(mp);