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.
diff --git a/setup.cfg b/setup.cfg
index 2a9acf1..046436b 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,2 +1,9 @@
 [bdist_wheel]
 universal = 1
+
+[pytype]
+python_version = 3.6
+inputs =
+    .
+exclude =
+    tests/