chore(tooling): experiment with `mutmut`

[danceratopz]

We should play around and see if we can make use of this mutation testing library:
https://mutmut.readthedocs.io/en/latest/

[felix314159]

Instead of first picking a dependency and then trying to find a usecase for it, we should wait until we encounter a problem then try to fix it ourselves and only if there is no other option explore dependencies that can help us. Please re-open if there is a critical problem that can only be solved by adding mutmut

[danceratopz]

@souradeep-das has experimented with mutmut and is in the process of adding it to the execution-specs CI 🥳

Here's the related issues and PRs:

• Investigate source code mutation testing execution-specs#1145
• Integration Mutation Testing on CI execution-specs#1362
• Update tests to align with mutation testing runs execution-specs#1361
• Run mutation tests in CI for changed files execution-specs#1364