Current pinned mypy version is incompatible with python 3.8+

[kclowes]

What is wrong?

Can't install dev dependencies of py_ecc with python3.8+ because the mypy version we're using requires a version of typed-ast that is incompatible with python 3.8. See issue: https://github.com/python/typed_ast/issues/126

How can it be fixed

Workaround for local development for now is to use python 3.7.

I started down the path of updating to mypy v0.910, but there were too many typing errors that needed more time than I had today. The branch I started is here.