Test running dmypy compiled on Windows before a release

[JukkaL]

dmypy was broken in the 0.710 release on Windows and our CI didn't catch it. See #7031 for the issue. It would be good to have CI find issues like this. Alternatively, we could add an extra step to our release process, but that would be unfortunate unless we can automate it.

[emmatyping]

So I suppose the best thing to do would be to conditionally build and run the tests on dmypy on tags, as that would not require running the test suite twice every commit. I can work on that this weekend.

[ilevkivskyi]

@ethanhs I think this makes sense.