Message 401423 - Python tracker

I did some experiments using the connection object as a "backref" in the callback context, but it seems that the GC does not play well with such ref circles; I ended up with a lot of ref leaks (yes, I modified the traverse and clear slots to visit and clear the backref).

Using the module object as a "backref", however, worked swell. It ends up being a much larger PR, though.