Implement a mypy plugin

[sobolevn]

MyPy is very useful when detecting @abtractmethod definitions that are unfilled.
I don't think that there's a clean way to do that with just type-hints. Maybe this can be added via a plugin?

[antonagestam]

Yes, a mypy plugin would be great and I would most likely use it myself. I think it could also work as a reference implementation for suggesting standardizing Abstract in the standard library.