Restore CI on Python 3.6

[rezib]

CI on Python 3.6 have been disabled in 0385953 due to ubuntu 20.04 runners support being dropped in GitHub Actions, and this version of Python is not supported in more recent version of ubuntu images. There is a feature request to add support of Python 3.6 in ubuntu 22.04: actions/setup-python#1048

There is also ongoing work in easybuild for this purpose: easybuilders/easybuild-framework#4790

Let's see what happen there!