GitHub Actions: Test on Python 3.13

[cclauss]

No description provided.

[john-hen]

The tests failed because we (still) include Python versions all the way down to 3.6, and that's not available on ubuntu-latest. It is on ubuntu-20.04, which is why that was there. And, as I learned from the error message of the workflow run, that Docker image has Python 3.13 as well.

I used to care about Python 3.6 at some point, as I was using Flake8 in CI for a project that needed 3.6. But that's no longer the case. So would also be open to dropping unsupported Python versions in CI here, like 3.8 and below.

[cclauss]

Dropped unsupported versions of Python.

[john-hen]

Thanks!