Have you heard of kallithea [1]? Not sure if it be what you want though because it also supports mercurial (and uses it for its development). But iirc, it's written in python.
One of the dealbreakers for me with Kallithea at this point of time though is the lack of an issue tracker. I think their main reason for not having it was that they don't want to impose one when you could be using one already.
But I would like it all integrated nicely with the git web interface.
Thank you.