add custom GitHub Actions workflow to run tests with Python 3.6 in Ubuntu 20.04 container

[boegel]

No description provided.

[Flamefire]

I also started working on the before in #4789 using the Python versions available from the action I also opened an issue with them to support Python 3.6 on Ubuntu 22+

I also had to remove a job that you might want to add here:

          # There may be encoding errors in Python 3 which are hidden when an UTF-8 encoding is set
          # Hence run the tests (again) with LC_ALL=C and Python 3.6 (or any < 3.7)
          - python: 3.6
            modules_tool: ${{needs.setup.outputs.lmod8}}
            lc_all: C

[Flamefire]

I added PRs for all repos to remove or replace the Ubuntu 20.04 runners:

• Update GitHub Actions config #4789
• Update GitHub actions runners to avoid using ubuntu-20.04 easybuild-easyconfigs#22459
• Update GitHub actions runners easybuild-easyblocks#3652

It also needs updating of the required PR-CI tests