Build Mercurial

Currently, Mercurial is installed by Heptapod Docker, with a dance involving installing build dependencies and removing them right away to avoid making the resulting Docker image even heavier.

After heptapod-docker#10 (closed) is done, Omnibus Heptapod could take care of the Mercurial build, and Heptapod Docker wouldn't need to do it any more.

That involves of course using the Python 3 version built by Omnibus, whereas up to now we've precisely avoided it. Things are different now, because the Omnibus Heptapod fork gives us the kind of control the lack of which led us to use a separate Python in Heptapod Docker.

I'm not sure if it's worth the effort and potential complication to also build Mercurial for Python2. Opinions welcome!