dev-vcs/gitg: add python3.6 compat

dev-vcs/gitg: add python3.6 compat

Works as well as with python3.4 (read: fails to actually import GitgExt override
with an assert failure). I would outright remove the python override support, as
there are no consumers of it, but that's a too big change for this revision and
we need to do something soon about libgit2-0.27 compat anyways..

Package-Manager: Portage-2.3.28, Repoman-2.3.9