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.

Author: rchen152

nox.py

@@ -15,7 +15,8 @@
 from __future__ import absolute_import
 import os
 
-import nox
+# https://github.com/google/importlab/issues/25
+import nox  # pytype: disable=import-error
 
 
 @nox.session
@@ -108,6 +109,19 @@ def lint_setup_py(session):
         'python', 'setup.py', 'check', '--restructuredtext', '--strict')
 
 
+# No 2.7 due to https://github.com/google/importlab/issues/26.
+# No 3.7 because pytype supports up to 3.6 only.
+@nox.session
+def pytype(session):
+  """Run type-checking."""
+  session.interpreter = 'python3.6'
+  session.install('.',
+                  'grpcio >= 1.8.2',
+                  'grpcio-gcp >= 0.2.2',
+                  'pytype >= 2018.9.26')
+  session.run('pytype')
+
+
 @nox.session
 def cover(session):
     """Run the final coverage report.

setup.cfg

@@ -1,2 +1,9 @@
 [bdist_wheel]
 universal = 1
+
+[pytype]
+python_version = 3.6
+inputs =
+    .
+exclude =
+    tests/