Add type annotations via MonkeyType

[bhrutledge]

Following @carljm's comments on #231 and #466, and watching his PyCon 2018 talk, I decided to back up a bit, and experiment with using MonkeyType to generate the annotations based on the test suite.

$ source .tox/py37/bin/activate
$ pip install monkeytype mypy
$ monkeytype run -m pytest tests
$ monkeytype apply twine.repository
$ git commit -am "Apply monkeytype to twine.repository"

There was some non-trivial clean-up (e.g., overly-specific Unions, replacing package: stub with package: PackageFile, etc.), but overall I found it to be straightforward and educational re: type hinting in general and the Twine codebase.

Also, mypy is passing at the moment, so I stopped ignoring the result in testenv:lint.

As before, I'm curious what folks think.

[bhrutledge]

This should be rebased/redone (and maybe closed and resubmitted) once #498 (Twine 2.0) is merged.

[bhrutledge]

After rebasing on master, this is now passing, with initial annotations for twine.package and twine.repository. I'm planning to add more type coverage, though I'm not sure how complete it can/should be before this is merged.

[di]

I think this is fine to merge as-is!

[bhrutledge]

Thanks!

[bhrutledge]

Also, thanks @waseem18 for your initial work on this!