Subj

This drops coverage for setup.py

Either we:

1. make our own docker images to test on (not sure how this works)
2. have the build take 6+ minutes instead of 1 (remove Mono 6.12 and reinstall 5.20 instead)
3. or switch Mono to 6.12 for testing

Considering we are doing a major version bump, we can pick option 3 for simplicity. @filmor thoughts?

I'm in favour of just using the latest version.