gh-151377: Fix races updating type slots and subclasses in free-threa…

[lpyu001]

In free-threaded builds, a newly created heap type can become visible through its base type’s subclasses dictionary after PyType_Ready(). Updating its slot dispatchers after that point must not write slot pointers unsafely while other threads may observe or update the type hierarchy.
This change makes fixup_slot_dispatchers() hold TYPE_LOCK in free-threaded builds, queue the required slot updates, and apply them with the world stopped. It also protects removal from base tp_subclasses during type deallocation with TYPE_LOCK.
Additionally, the debug-only _Py_TYPE_REVEALED_FLAG is read and written from different threads by type-safety assertions, so its accesses are changed to atomic operations to avoid TSAN reports on the debug marker itself.

• Issue: Data races on type slots and tp_subclasses between type_setattro and type_new / type_dealloc #151377

[lpyu001]

Tested on Ubuntu 24.04，did not observe TSAN warnings.