Add stubs for importlib.util

[brettcannon]

Closes #189

[gvanrossum]

Great! It's entirely up to you to double-check that these stubs actually work -- neither mypy's test suite nor typeshed's verifies that a stub matches CPython. It would be hard to imagine how that would work anyways -- so we rely on people writing real code that uses a module and annotating it as a way to verify that the stubs are correct. I assume this won't be much of a challenge for you! But given the amount of code you just landed for #189 there is bound to be a bug somewhere, and it may take a while before someone runs into it. Godspeed!

[brettcannon]

Is there an easy way to run mypy against Python's test_importlib test suite from a CPython checkout? That's probably the most comprehensive use of importlib that I know of.

[gvanrossum]

You could just copy that package somewhere else and run mypy on it. But you'd probably first have to add annotations to the tests, which might be a huge task by itself. Also the use of mocks might possibly cause problems.