Add stubs for some missing imports

[bhrutledge]

Related to #231

Okay, this is definitely yak-shaving, but I was curious how hard it might be to add our own stubs, and what value they might bring. If nothing else, I found it to be an informative exercise, and thought others might as well. Before I go further, I'm curious if there's any appetite for merging something like this.

Changes:

• Add stubs for the bits of the pkginfo and importlib_metadata API that twine uses
• Fix resulting mypy errors
• Add formatting and linting for stubs (incomplete)
• Type coverage: https://twine-mypy.now.sh/

+-------------------------+-------------------+-------------------+
| Module                  | Before            | After             |
+-------------------------+-------------------+-------------------+
| twine                   |  22.73% imprecise |   2.27% imprecise |
| twine._installed        |  10.00% imprecise |   8.33% imprecise |
| twine.cli               |  17.81% imprecise |  13.51% imprecise |
| twine.package           |  17.10% imprecise |   1.48% imprecise |
| twine.repository        |  16.73% imprecise |  16.34% imprecise |
| twine.wheel             |   7.61% imprecise |   2.15% imprecise |
| twine.wininst           |   3.28% imprecise |   1.64% imprecise |
+-------------------------+-------------------+-------------------+
| Total                   |   8.95% imprecise |   5.99% imprecise |
+-------------------------+-------------------+-------------------+

TODO:

• Review typeshed/CONTRIBUTING.md for stub code style
• Rework linting/formatting for stubs. I don't think the flake8 configuration is right. I tried flake8-pyi, but saw some inconsistent behavior, though it might be worth a closer look.
• Maybe: Add stubs for more missing imports

[bhrutledge]

Since nobody has said this is a great idea, and it's probably more than we want to maintain, I'm closing this. We can revive it later if there's interest.