Add type-checking via pytype to api_core. (#6116)

* Add pytype to api_core's setup.cfg.

With these changes, pytype can now be run on api_core with the
following steps in a Python 3.5 or 3.6 virtualenv:

$ git clone git@github.com:GoogleCloudPlatform/google-cloud-python.git
$ pip install googleapis-common-protos protobuf google-auth requests \
  setuptools six pytz futures grpcio grpcio-gcp
$ pip install pytype
$ cd google-cloud-python/api_core/
$ pytype -V3.5  # or 3.6

* Add a pytype session to nox.py.

* Run pytype for python 3.6 only.
2 files changed