Fix CI problems

[dwhswenson]

Problems with CI that this will fix:

1. We're way behind on Python versions; this brings the CLI in line with versions tested in the OPS library
2. We're having a problem with the build process; seems something was problematic with the pip install stack (I think this was fixed by not trying to support older Pythons)
3. Tests needed to be updated from setup/teardown to setup_method/teardown_method
4. Updated CodeCov, as it didn't seem to allow uploads without token.

This wasn't as bad as I feared!

[dwhswenson]

This is ready for review and comment. Without review, I will merge this after at least 24 hours, no sooner than Wed 24 Apr 17:00 GMT (12:00 my local).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b430d8b) to head (e7f4149).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #74   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          110       110           
  Lines         5416      5416           
=========================================
  Hits          5416      5416           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sroet]

LGTM