I took it out because it was always lagging behind changes in core hg, and its dependencies broke stuff. But it should be easy enough to install by simply running py -2 -m pip install hg-git --user if you have python2 installed on your system. (It can't be py3 because it is put into a py2 specific location.) See #5597 (closed) (and an earlier issue I referenced in that thread). Since it now seems to be getting regular maintenance again, I wouldn't mind adding it back into the (not-yet-finished) py3 release.
I am currently using it with python 3 and a plain "pip install mercurial hg-git" and it just works. For some reason on a couple of windows machines this fails (because windows) and thg in this cases ships a working hg -- alas modulo the broken hggit currently shipped.
I am 100% sure that hg-git + dulwich these days are perfectly py3 compatible and latest-hg compatible (a lot of work went into dulwich and hggit recently).
I can confirm that hg-git 0.10.x works fine with current Mercurial, is overall more stable than previous versions and is compatible with both py2 and py3 (py2 might still be of importance for thg). A big 👍 on our side for inclusion in TortoiseHg
Note that some recent hg-git minor features are unsupported under py2, such as .git-credentials file, but it doesn't compromise the overall operation.