Investigate mutation testing for improved test validity

[Mutation testing](https://en.wikipedia.org/wiki/Mutation_testing) is a principle of "testing one's tests". In short, what it does is runs a CI test suite against a mock 'broken' version of the code; for example, it will change a `<` sign in the code to a `>` sign, a `*` to a `/`, change literal values, or change keywords like `is` to `is not`, `break` to `continue`, etc. If this 'broken' code still passes the CI tests, that suggests the tests need to be improved.

Implementing this is incredibly easy; [packages like `mutmut`](https://mutmut.readthedocs.io/) will simply run on top of pytest, both running the test suite and comparing it to a mutation test. Investigate `mutmut` for MDMC's cron testing, and see whether it provides valuable insight into the validity of our tests.