The path towards great CI

[MartinThoma]

I want the CI to cover so much that I can pretty much rely on it when I have to make a decision if a new feature gets merged.

My plan is to improve CI in roughly those steps:

1. Line Coverage: Get close to 100%. If a line isn't covered, we don't have any chance to discover bugs in that line.
2. Mutation Testing: 100% line coverage can still leave a lot of bugs in there. If we kill 100% of the mutants that shows that our tests are actually catching typical mistakes. I use mutmut for mutation testing
3. Performance Testing: Performance Testing #761
4. Fast CI: The quicker the CI is, the faster contributors can see issues in their PRs and fix it without a context switch.

There are a couple of things CI could also do to make my live easier:

5. Create a GH action that just automatically creates a GH release on a new git tag #748
6. Automatically create the changelog
7. Automatically release on PyPI (I'm not sure yet how I feel about that)