[Python-3000] PEP 3121 implemented

["Martin v. Löwis"]

> So, next question: what's the migration path to write portable code, then? Up
> to this change, we supported every Python version starting from 2.3, including
> 3.0alpha5, and it would be nice to continue to do so with 3.0 final.

I would split out the main body of the initialization into a separate
function, and then have both init<foo> and PyInit_<foo> call that
function.

Regards,
Martin