[Twisted-Python] mypy integrated with CI for twisted

[Craig Rodrigues]

I have merged some more fixes for mypy to Twisted trunk branch.

In trunk, you can run mypy with:

*tox -e mypy*

Currently this results in *171* errors, which is way down from >1000 errors
a month ago.

In addition, if you look at any new PR's there is a *Mypy Ubuntu* job
running on Azure pipeline, which runs mypy.  Right now errors from this job
are ignored and does not block the PR.  However, if we can get the mypy
errors down to zero, we can make mypy status a blocker for the PR.

--
Craig

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