update constraints file to include pinned deps

Author: ohmayr

packages/google-cloud-pubsub/testing/constraints-3.9.txt

@@ -1,9 +1,13 @@
 # -*- coding: utf-8 -*-
 # This constraints file is required for unit tests.
 # List all library dependencies and extras in this file.
-google-api-core
-google-auth
-grpcio
-proto-plus
-protobuf
-grpc-google-iam-v1
+google-api-core==1.34.1
+# cryptography is a direct dependency of google-auth
+cryptography==38.0.3
+# TODO(https://github.com/googleapis/gapic-generator-python/issues/2453)
+# Add the minimum supported version of grpcio to constraints files
+google-auth==2.14.1
+grpcio==1.51.3
+proto-plus==1.22.3
+protobuf==3.20.2
+grpc-google-iam-v1==0.14.0