[CI] Add test coverage reporting

[ByteOtter]

What this PR does / why we need it:

This PR adds automated test coverage reporting via Codecov.
During development it is easy to lose sight of keeping acceptable test coverage. This new CI will help with that by automatically running pytest-cov and reporting test coverage back to the contributors.

It includes:

• A Web Interface that can be accessed by all organization members with which we can track current and past coverage rates
• A bot that automatically reports its findings back to the contributor as a PR comment, including annotations.
• Configurable failure rate (now set to 50%) to increase acceptable coverage standards as we fill in gaps.

Which issue(s) this PR fixes:
Fixes: #139
Tracks: #138

Special notes for your reviewer:

For this change to take effect, the organization admin has to connect the organisation (or a specific repo from it) to Codecov and create an upload token. Information about how to do this is here: https://docs.codecov.com/docs/quick-start

Release note:

NONE

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[yeoldegrove]

https://app.codecov.io/gh/gardenlinux/python-gardenlinux-lib/pull/142/tree would be the coverage report for this PR.
We'll discuss in the team if we want to have enable codecov.

[ByteOtter]

I think I have now updated the workflow with all concerns in mind, and wil be splitting the other changes off into actions/setup as we discussed

[NotTheEvilOne]

LGTM

[ByteOtter]

Thanks for your help I'll clean the git history and merge it :)