[Python-3000] Accessing module state from extension types

["Martin v. Löwis"]

> How is this supposed to work?

The design was that you use PyState_FindModule, as an efficient way for
getting a module object if you have the module def. The implementation
fills an index into the module def (which will stay constant across
interpreters), this this should give you your module object anywhere,
in constant time.

If you have specific proposals on how to make this more convenient to
use, please go ahead. (also, if you think that this somehow flawed:
this would be the time to mention it)

Regards,
Martin