Add support for mypy

[Hnasar]

It looks like mypy has support for outputting Cobertura reports: https://github.com/python/mypy/blob/master/mypy/report.py#L481

Maybe it'd be trivial to add support to mypy for diff-cover and diff-quality?

[Bachmann1234]

If Mypy outputs Cobertura then you can already use diff-cover! You just use that mypy coverage file. In theory I think it would all just work

[blueyed]

Yeah, it should work already. Working on this for pytest in https://github.com/pytest-dev/pytest/pull/6486. This issue can be closed.

[blueyed]

Maybe adding support for --violations=mypy might make sense though (https://github.com/Bachmann1234/diff_cover#adding-diff-quality-support-for-a-new-quality-checker), but that would have to be done for mypy itself then.