[3.13] gh-117657: Enable test_opcache under TSAN (GH-129831) (GH-130597)

Fix a few thread-safety bugs to enable test_opcache when run with TSAN:

 * Use relaxed atomics when clearing `ht->_spec_cache.getitem`
   (gh-115999)
 * Add temporary suppression for type slot modifications (gh-127266)
 * Use atomic load when reading `*dictptr`

(cherry picked from commit f151d271591ec525eaf01fa7b128e575374888b9)