ci: tweak mypy to check 'google.protobuf' (#310)

Install the 'types-protobuf' package in support.

Author: tseaver

google/__init__.py

@@ -21,4 +21,5 @@
 except ImportError:
     import pkgutil
 
+    # See: https://github.com/python/mypy/issues/1422
     __path__ = pkgutil.extend_path(__path__, __name__)  # type: ignore

mypy.ini

@@ -2,6 +2,3 @@
 python_version = 3.6
 namespace_packages = True
 ignore_missing_imports = True
-
-[mypy-google.protobuf]
-ignore_missing_imports = True

noxfile.py

@@ -170,7 +170,9 @@ def pytype(session):
 def mypy(session):
     """Run type-checking."""
     session.install(".[grpc, grpcgcp]", "mypy")
-    session.install("types-setuptools", "types-requests", "types-mock")
+    session.install(
+        "types-setuptools", "types-requests", "types-protobuf", "types-mock"
+    )
     session.run("mypy", "google", "tests")