I've moved the result = PyImport_NotifyPostImport(result); inside the
protected block. It's now protected by the import lock. I've also added
the lock protection to the register function.

The notify method is now exposed through the imp module, too.

I've checked multiple code paths. They all end up in 
PyImport_ImportModuleLevel(): builtins.__import__(), PyImport_Import(),
PyImport_ImportModule(). Only PyImport_ImportFrozenModule() doesn't use
the code path in imp_init_frozen().