The only environments left on AppVeyor are now 2.7, 3.5, and 3.6. The remaining Python versions are now also tested on Windows using GitHub Actions.

Additionally, a new group of dependencies called "oldest" is now tested as well to ensure compatibility with the oldest pinned versions in requirements.txt and setup.py.

AppVeyor is used for 3.5 and 3.6 because those versions aren't available on GitHub Actions at all. Python 2.7 crashes on Windows with a pip cache failure so it only runs on GHA with the "oldest" dependencies on Ubuntu.

The other Python version running "oldest" dependencies is 3.7, also only running on Ubuntu

This supersedes #320.