Add pytest-subtests to dev requirements This pytest plugin adds support for unittest's TestCase.subTest (for Python >= 3.4). Since we we now only support python3 and since we use a fair amount of these, let's benefit of better reporting by using this plugin.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances