I had been hoping for something like this.
Not having to dig around to find an external host for mailing lists for my projects seems like a good idea in general.
mailman is very well-known as mailing-list-manager and used by the vast majority of mailing lists these days. If you insist on something written in python, I'd suggest to go with it.
Being able to download the archive in mbox format would also be nice.
Yes, please. I just created a list on Librelist.com but I would have preferred to host it here right next to the source code — just imagine nice things like the archive linking back to commits when it recognized a changeset hash...
Some kind of discussion feature would be very much appreciated. I would even be willing to use the current issue tracker as a pseudo-forum, if I were only able to add my categories, etc. to support it.