[Twisted-Python] mypy now mandatory for Twisted pull requests

Hi, In the Twisted trunk branch, I have reduced the number of errors reported by mypy to *zero!* You verify this by running: *tox -e mypy* In addition, I have worked with Adi to enable CI for Twisted checkins so that mypy is run for each pull request. Adi configured the GitHub repo to *require* mypy to pass in order for a Twisted pull request to be merged. Thanks to Adi, Wilfredo, Tom, and Maarten for helping with various mypy-related code reviews! -- Craig