git-cinnabar calls into Mercurial as a library, so we make use the
same version of Python as Mercurial.
Support for Python 3 is an experimental feature in git-cinnabar, but
we unconditionally use it here because:
* Mercurial in Nixpkgs only supports Python 3.
* git-cinnabar touches a network, and for that purpose it's good to
use a version of Python that is going to get security updates.
