This also makes the flask stubs that depend on click parseable.
* In pytype_test, only parse files with the .pyi extension.
(There's a README in third_party/2and3/click/.)
* Annotate with ContextManager rather than using the
contextmanager decorator. This is shorter, removes a
dependency, and gets rid of the slight weirdness of a
decorator needing to be evaluated in a stub.
* Fix non-stub things.