Continuous integration tests

[shoyer]

What should these look like?

At the least, we probably should verify that mypy can successfully type-check some simple examples using these type annotations.

[henryJack]

Happy to write the example annotated code once the stubs are done. It's probably best to add a mypy job to the pipeline with multiple scripts testing that mypy passes and throws errors where it's supposed to.

[eric-wieser]

I'm workin on #2, and I'm not even sure I'm able to test this locally - it seems mypy doesn't support _stubs packages yet.

[shoyer]

It seems that we are very cutting edge here in relying on PEP 561. @ethanhs is still finishing up the mypy implementation in python/mypy#4278, but I guess we could test locally against his branch.

[shoyer]

See python/typeshed#754 and python/typeshed#1339. It seems like tests for stubs make sense for at least tricky cases, but are probably overkill when required for everything.

[eric-wieser]

We're probably going to want to use whatever mypy is using to run its own tests here. Perhaps to support PEP561, mypy should extract its testing module to be standalone

[emmatyping]

The branch is in a somewhat limited state, it checks against packages installed only for the Python mypy is installed in. Just a heads up. It should work for the simple case.

[shoyer]

We have this now.