Drop support for Python 3.6 in GitHub workflows

[bemoody]

Python 3.6 is officially end-of-life and is no longer supported in GitHub workflows on ubuntu-latest.

I guess that this is the case since ubuntu-latest was updated to Ubuntu 22.04: https://github.com/actions/runner-images/issues/6399

Alternatively, if folks want to continue supporting Python 3.6 in this package, the test suite could be changed to use an older Ubuntu release.