Enable testing for Python 3.13

Author: parthea

.github/sync-repo-settings.yaml

@@ -25,4 +25,5 @@ branchProtectionRules:
     - 'unit (3.10)'
     - 'unit (3.11)'
     - 'unit (3.12)'
+    - 'unit (3.13)'
     - 'cover'

testing/constraints-3.13.txt

@@ -0,0 +1,6 @@
+# -*- coding: utf-8 -*-
+# This constraints file is required for unit tests.
+# List all library dependencies and extras in this file.
+google-api-core
+proto-plus
+protobuf