Log Message:
py-grpcio: updated to 1.20.0

Release v1.20.0
Add Python example for error handling.
python docs: details are UTF-8 encodable, not just ASCII.
Use C-Core API to perform time conversion.
Use correct C integer types in Cython.
Add the missing grpc_cfstream dependency.
Strip Python wheel binary.
Simplify batch operations event interpretation.
add enter_graceful_shutdown() to health service.