First fork the repository and clone your fork.
Install the dependencies:
pip install -r requirements.txt -r test_requirements.txt
pytest
This also runs E2E tests that verify that mutmut run produces the same output as before. If your code changes should change the output of mutmut run and this test fails, try to delete the snapshots/*.json files (as described in the test errors).
If pytest terminates before reporting the test failures, it likely hit a case where mutmut calls os._exit(...). Try looking at these calls first for troubleshooting.
You can install your local version of Mutmut and run it, including any changes you have made, as normal.
python -m pip install --editable <path_to_mutmut_codebase>