CI test commands inconsistent across distros

The test commands run in .gitlab-ci.yml aren't consistent across all distros.

For archlinux, debian and fedora it runs python3 setup.py test. For gentoo it runs python3 -m unittest discover -v.

Running python3 setup.py test prints following warning in archlinux (it doesn't for fedora and debian):

WARNING: Testing via this command is deprecated and will be removed in a future version. Users looking for a generic test entry point independent of test runner are encouraged to use tox.

I tested myself python3 -m unittest discover -v doesn't print this warning there.

Is there a reason not using python3 -m unittest discover -v everywhere?