Bumped Python to 3.8.7

Closes #16

Also done in the heptapod-base CI image so that we'll have a few test
runs using it before 0.18.1 (HGitaly "current" and RSpec).

--HG--
branch : heptapod-stable
5 jobs for !21 with topic/heptapod-stable/python-3.8.7 in 16 minutes and 57 seconds (queued for 1 second)
merge request