Pin grpcio and grpc-tools to 1.31.0

Can remove when https://github.com/grpc/grpc/pull/24094 is in a patch
release

tools/run_tests/helper_scripts/prep_xds.sh

@@ -20,7 +20,7 @@ cd "$(dirname "$0")/../../.."
 
 sudo apt-get install -y python3-pip
 sudo python3 -m pip install --upgrade pip
-sudo python3 -m pip install grpcio grpcio-tools google-api-python-client google-auth-httplib2 oauth2client
+sudo python3 -m pip install grpcio==1.31.0 grpcio-tools==1.31.0 google-api-python-client google-auth-httplib2 oauth2client
 
 # Prepare generated Python code.
 TOOLS_DIR=tools/run_tests