Install missing stubs for mypy 0.900

[bhrutledge]

Prompted by the output from a failed test run:

types run-test: commands[0] | mypy --html-report mypy --txt-report mypy twine
twine/utils.py:25: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
twine/__main__.py:20: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
twine/repository.py:18: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
twine/commands/upload.py:19: error: Library stubs not installed for "requests" (or incompatible with Python 3.9)
twine/commands/upload.py:19: note: Hint: "python3 -m pip install types-requests"
twine/commands/upload.py:19: note: (or run "mypy --install-types" to install all missing stub packages)
twine/commands/upload.py:19: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

The suggested docs and relevant blog post seemed clear enough. I tried mypy --install-types, but it's interactive (for now):

% mypy --install-types
Installing missing stub packages:
/Users/bhrutledge/Dev/twine/.tox/types/bin/python -m pip install types-requests

Install? [yN] 

[bhrutledge]

Hm. Codecov is failing randomly. Will investigate later.