Add support for Python 3.10-3.11, drop EOL 2.7-3.6

[hugovk]

Previously pytest 3.6+ were supported.

Python 3.10 and 3.11 requires pytest 6+, so they can only be tested on pytest 6 and 7. Because we're dropping Python 2 and only support modern Python, I suggest we simplify to only support modern pytest and support the last two versions.

Also test on GitHub Actions. I didn't remove .travis.yml, as I don't know if it's still working - also it has the PyPI deploy stuff for reference if that needs moving over too.