Review mutmut configuration

[stefan6419846]

We currently have a configuration for mutmut (https://mutmut.readthedocs.io/en/latest/index.html) inside the repository, but do not seem to really use or have a look at it. This does not really make much sense and should be reviewed to decide on how to further deal with this.

As of today, mutmut generates 40147 possible candidates - some of which do not make much sense and thus require further configuration (see screenshot). As we run all Python tests as well as mypy for each mutation (see mutmut-test.sh), this tends to require quite some time and resources. (If I manage to get hold of a completed run, I will report back the relevant numbers, although this probably is rather unlikely as given a hypothetical duration of one minute per mutation, this would still take 28 days.)