I was just about to bring up mutation testing. I've had some pretty great success with PIT [1] when writing Java. Code coverage and mutation test coverage pair wonderfully together.
[1]:https://pitest.org [1]:https://pitest.orgFor Python we use mutmut [1].
I think it's pretty important that we who know about MT and have used it keep talking about it. It's a pretty great tool that more people should have in their tool belt.