[Python-3000] PEP 3121 implemented

["Martin v. Löwis"]

> Was it necessary to remove Py_InitModule4() for this change? We use that in
> Cython, so this breaks building extension module in current Py3.

Yes: the new module API takes a number of additional function pointers,
which can't be passed in through Py_InitModule4.

While the change indeed systematically breaks all modules, this breakage
is shallow: it's usually straight-forward to port a module to 3.0 with
little changes to the init function.

Regards,
Martin