WIP: mypy support

[bhrutledge]

Related to #231, building on #344.

I'm a newbie to mypy, but at @brainwane's suggestion, I decided to dive in as an exercise. I've skimmed the Zulip blog (which is somewhat outdated, e.g. CLI options and broken links), read mypy's getting started and existing codebase docs, and reviewed the existing PR's.

I got lint-mypy to pass in 8f569a5 (covering some of the ground from #359), then added check_untyped_defs as suggested by Zulip. I know the preference is to use stub files, but the initial batch of errors seemed to require local variable annotations. I'd love some feedback on this direction.

Current errors:

twine/wheel.py:56: error: Item "None" of "Optional[Match[str]]" has no attribute "group"
twine/utils.py:216: error: Module has no attribute "get_credential"
twine/utils.py:241: error: Module has no attribute "get_password"
tests/test_check.py:27: error: "Callable[[str], int]" has no attribute "calls"
tests/test_check.py:38: error: "Callable[[str], int]" has no attribute "calls"

[brainwane]

@carljm is there some kind of "hey mypy experts! help?" bat-signal I can shine? If so, help me shine it? :-)

[carljm]

@brainwane I commented on #231 with some thoughts about overall approach to typing in twine. As for bat signal, a lot of Python typing experts hang out in https://gitter.im/python/typing -- you could ask specific questions there for sure, you could also post a link to this PR and see if anyone is interested in reviewing it.

[codecov]

Codecov Report

Merging #466 into master will decrease coverage by 0.19%.
The diff coverage is 90.9%.

@@            Coverage Diff            @@
##           master     #466     +/-   ##
=========================================
- Coverage   82.93%   82.73%   -0.2%     
=========================================
  Files          14       14             
  Lines         750      753      +3     
  Branches      108      108             
=========================================
+ Hits          622      623      +1     
- Misses         92       94      +2     
  Partials       36       36

Impacted Files	Coverage Δ
twine/utils.py	82.39% <100%> (-1.3%)	⬇️
twine/repository.py	73.58% <100%> (+0.25%)	⬆️
twine/commands/__init__.py	100% <100%> (ø)	⬆️
twine/package.py	86.84% <80%> (+0.11%)	⬆️
twine/wininst.py	31.57% <0%> (ø)	⬆️

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[bhrutledge]

Note: there's an alternate approach in #469, leveraging MonkeyType.

[bhrutledge]

Closing in favor of #469, assuming #437 will be merged.