Skip mypy on pynightly if numpy-1.20.0-dev0... is used (#40656)
Summary:
Also modernize the test script itself by using `mypy.api.run` rather than `subprocess.call`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40656
Differential Revision: D22274421
Pulled By: malfet
fbshipit-source-id: 59232d4d37ee01cda56375b84ac1476d16686bfe
