Hello. Upstream currently doesn't officially provide Python 3 support. It seems that adding Python 3 support isn't very problematic, see URL. While upstream is concerned about support for Python < 2.7, we don't have those versions in Gentoo anymore. Could you please add Python 3 support to sphinxtogithub either based on the PR in URL or by other means? This is required at least for git-cola[doc], which already has Python 3 support.
Get it merged upstream first please.