py36: skipped because could not find python interpreter with spec(s): py36

[hroncok]

Since the update to Fedora 42 (and hence virtualenv 20.29), testing with Python 3.6 doe snot work out of the box. The CI here does not fail because the Python 3.6 environment is skipped:

py36: skipped because could not find python interpreter with spec(s): py36
py36: SKIP ⚠ in 0.03 seconds

See https://tox.wiki/en/stable/faq.html#testing-end-of-life-python-versions

We should add a new CI job that tests with py36m using the method described in tox docs.