[Pymilter] python3 support?

[Stuart D Gathman]

On Nov 10, Tom Hendrikx transmitted in part:

> Is there any interest in getting pymilter to work with python3?

I took a brief look.  Will have to change all the PyString to PyBytes or 
PyUnicode.  I'm leaning toward splitting the py3 version (not trying
to maintain 1 source and running 2to3 as part of build).  That will
let me make a few incompatible changes to the API.  For instance,
getting rid of all the set_XXX_callback module methods and likely using
keyword args on register() as I did with the newer callbacks.

So I created a python3-branch tag.