_PyType_GetModuleByDef (added in bpo-42100)allows module state access 
from slot methods (like tp_init or nb_add),
the main thing missing from PEP 573 (Module State Access from C 
Extension Methods).
It's time to make it public.

The function itself can be implemented using only limited API, though 
it's a bit tricky to do so correctly (and our implementation uses 
private speedups), so it's better if extension authors can use it as a 
pre-made building block.

Discussed in: https://mail.python.org/archives/list/capi-sig@python.org/thread/WMSDNMQ7A6LE6X4MQW4QAQUKDDL7MJ72/
Note that a bug was found in the CPython optimization recently: bpo-46433