GitHub is removing Ubuntu 20.04, and Ubuntu 22.04 doesn't have Python
3.6 that we want to continue testing for. Switch to using containers
instead so we have more control over when to deprecate a Python version.
pypi has changed something in their end that is incompatible with older
Python versions. Fortunately, anything older than 3.6 is no longer that
interesting as that is the new baseline set by the oldest enterprise
distributions.
GitHub has deprecated 18.04, so we can't continue running tests on that
old distribution. Unfortunately, we can't switch to the latest 22.04
since GitHub doesn't package anything older than Python 3.7 for that
runner.
And unfortunately the oldest version in 20.04 is Python 3.5, so we have
to remove the test runs for Python 3.4.
Pierre Ossman