* Moves the pytype installation to requirements-tests-py3.txt, now
that pytype can run under Python 3.5+.
* Changes tests/pytype_test.py to not require a --python{version}-exe
argument when it can automatically find the Python interpreter, and
cleans up a few typos and out-of-date things.
* Updates the appropriate documentation.
* Updates .travis.yml.
* Add parts of third_party/ to pytype_test.
Pytype uses six and mypy_extensions, so these pyi files should be
tested with pytype.
* Remove newly loadable pyi files from pytype_blacklist.
Rebecca Chen