Get CI tests working again, particularly macOS

[benhoyt]

These have stopped working, as you can see from this run. On both Windows and macOS it looks like 2.7 is no longer supported (maybe we should just remove the tests on 2.7). But all the macOS tests seem to be failing, seemingly due to ImageMagick not being installed on the latest macos runners. This can probably be fixed with a brew install imagemagick.