ci-py-heptapod: using a suitable hg-git py2 version

That Heptapod 0.12.3 tag is good enough to test with Mercurial 5.4
on Python2. Once we are done porting to python3, we'll simply
flip other that and the hgext3rd base image
1 job for topic/default/heptapod-0.13 in 15 seconds (queued for 1 second)