build-release/continuous integration: increase python version testing coverage

[dmsimard]

The github action that exercises build and installation tests currently runs on top of python 3.9: https://github.com/ansible-community/antsibull/blob/main/.github/workflows/antsibull-build.yml

We should increase installation testing coverage to other python versions as well, it would have allowed us to detect that Ansible 5 was not installable under python<3.8.

[felixfontein]

This means that the testing of the resulting build should run with Python 3.8, but not the build itself. The build and testing have to be split up into different workflows, the test ones depending on the build one.