gh-81057: Move PyImport_Inittab to _PyRuntimeState (gh-99402)

We actually don't move PyImport_Inittab.  Instead, we make a copy that we keep on _PyRuntimeState and use only that after Py_Initialize().  We also prevent folks from modifying PyImport_Inittab (the best we can) after that point.

https://github.com/python/cpython/issues/81057