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

[colesbury]

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

• Use relaxed atomics when clearing ht->_spec_cache.getitem (Make the specializing interpreter thread-safe in --disable-gil builds #115999)
• Add temporary suppression for type slot modifications (Type slots are not thread-safe in free-threaded builds #127266)
• Use atomic load when reading *dictptr
  (cherry picked from commit f151d27)

• Issue: Make TSAN tests pass with the GIL disabled in free-threaded builds #117657