Disallow races in initializing types

@kumaraditya303 looked a little at the races I reported against CPython here: https://github.com/python/cpython/issues/130421#issuecomment-2970633104

He tells me that the PyTuple_Pack and _Py_IncRef races are happening because it's possible for a supertype and subtype to initialize simultaneously. Right now CPython doesn't do anything to sanity check that situation and PyO3 should probably do something a little saner.

I think as a first pass we should consider replacing the GILOnceCell uses in the pyclass internals with e.g. OnceLock and OnceLockExt::get_or_init_py_attached. If that doesn't fix the races, we should also add checks when initializing subclasses to make sure that the superclass is initialized and wait until that's finished before trying to initialize a subclass.

Start by tracing the pyclass internals that use GILOnceCell, then compare their initialization paths with OnceLock and OnceLockExt::get_or_init_py_attached. Check whether simultaneous supertype and subtype initialization can still occur; done means the reported PyTuple_Pack and _Py_IncRef races are prevented and superclass initialization is ordered before subclass initialization.