py-hg-git alpha update

[Nikita Gillmann <nikita%n0.is@localhost>]

devel/py-hg-git has been broken for at least 6 months or how long we've
been building mercurial
with python3.7 by default, I never reported the breakage in gnats:
https://foss.heptapod.net/mercurial/hg-git/issues/310

0.9.0a1 is an alpha release but I'm working towards testing it locally.
If it works, would it be okay to
commit an update? Right now hg-git is unusable unless we provide an hg
which builds against python2.7,
which we don't do.