Pytype

* Support asynchronous generators.
* Fix variable annotations in Python 3.7.
* Fix a pyi circular import bug.

* Support future.utils.with_metaclass.
* Support coroutine annotation.

* Add support for unittest.TestCase.setUp().
* Open-source a user guide.
* Open-source the FAQ.

* Improve support and add testing framework for target Python 3.7.
* Handle files that generate parse errors more gracefully.

* Add rudimentary support for target Python 3.7.
* Make error messages for Python operators friendlier.
* Set the default target python version to the host version.
* Change default behavior to stop at first error and add a --keep-going option.
* Flag annotations with the wrong parameter count and invalid uses of NoReturn.
* Improve pytype and merge-pyi status messages.
* Support PEP-526 variable annotation syntax.

* Use --imports_info to allow preserving generated type stubs between runs.
* Handle TypeParameter inheritance more robustly with a union-find approach.
* Get more precise deps from importlab to allow build parallelization.
* Trim the dependencies of builtin and system files from the import graph.
* Generate a build.ninja file and call pytype-single via ninja.