CI: GitHub Actions Python 3.6 job failing

[Ken Lauer]

Looks like Python 3.6 is unavailable on GitHub Actions Ubuntu 22.04 (see the version manifest here: https://github.com/actions/python-versions/blob/main/versions-manifest.json and the issue here actions/setup-python#544)

CI could either be rolled back to 20.04 for the 3.6 job or potentially dropped entirely since 3.6 security support ended in 2021 (ref)

(Failing example in test suite of #513 - https://github.com/wyfo/apischema/actions/runs/3640787547/jobs/6145921372)