fix: add mypy checking + 'py.typed' file (#290)

noxfile.py

diff --git a/noxfile.py b/noxfile.py
index 617dc58..926f9f5 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -35,6 +35,7 @@
     "unit_grpc_gcp",
     "cover",
     "pytype",
+    "mypy",
     "lint",
     "lint_setup_py",
     "blacken",
@@ -155,6 +156,14 @@
     session.run("pytype")
 
 
+@nox.session(python=DEFAULT_PYTHON_VERSION)
+def mypy(session):
+    """Run type-checking."""
+    session.install(".[grpc, grpcgcp]", "mypy")
+    session.install("types-setuptools", "types-requests", "types-mock")
+    session.run("mypy", "google", "tests")
+
+
 @nox.session(python="3.6")
 def cover(session):
     """Run the final coverage report.