Make mypy a separate CI job

[TomNicholas]

The mypy checks are currently part of the same CI jobs that the test suite runs are. It would be better to have the mypy test be separate, so that it's easier to see if a PR only "failed" due to mypy rather than due to actual runtime test failures.

cc @norlandrhagen

[norlandrhagen]

I can take this one if you haven't started a PR @TomNicholas.

[TomNicholas]

That would be great, thanks @norlandrhagen

[norlandrhagen]

#254