Allow passing metadata as part of creating a bidi (#7514)

* allows providing rpc metadata for bidi streams
diff --git a/noxfile.py b/noxfile.py
index fbc51ee..06e78c1 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -88,7 +88,7 @@
 def pytype(session):
     """Run type-checking."""
     session.install(
-        ".", "grpcio >= 1.8.2", "grpcio-gcp >= 0.2.2", "pytype >= 2018.9.26"
+        ".", "grpcio >= 1.8.2", "grpcio-gcp >= 0.2.2", "pytype >= 2019.3.21"
     )
     session.run("pytype")