Core: Classify 503 Service Unavailable errors as transient. (#8182)

Also, pin grpcio < 2.0dev.

Closes #5410.

diff --git a/setup.py b/setup.py
index a82cf0e..d137450 100644
--- a/setup.py
+++ b/setup.py
@@ -39,7 +39,7 @@
     'futures >= 3.2.0; python_version < "3.2"',
 ]
 extras = {
-    "grpc": "grpcio >= 1.8.2",
+    "grpc": "grpcio >= 1.8.2, < 2.0dev",
     "grpcgcp": "grpcio-gcp >= 0.2.2",
     "grpcio-gcp": "grpcio-gcp >= 0.2.2",
 }