Mandate Python 3
In omnibus-gitlab!2650 (comment 90354386), it appears that we call python2 directly in https://gitlab.com/gitlab-org/gitlab-markup/blob/8dfe12783f4882883a2854f7f94ea333d937155f/lib/github/markups.rb#L36, but for Omnibus we have this manual patch in https://gitlab.com/gitlab-org/omnibus-gitlab/blob/25bf7026e047655bb1248a31514627e401078e7d/config/software/gitlab-rails.rb#L100-110 that bumps the version from 2 to 3.
This patch is only applied on GitLab Rails nodes, so this fails on Gitaly nodes.
@brodock Is there a reason we aren't just mandating Python 3 or auto-detecting which version is available? I see https://github.com/github/markup/pull/919/files was merged, so is this just a matter of updating code?