add lib-forward-testing-python3 test job

This is a mechanically generated patch to add a functional test job
running under Python 3 as part of the python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: If3765f1d4fa236e37c48a68bcd1093addfc751ff
Story: #2002586
Task: #24304

.zuul.yaml

@@ -6,6 +6,7 @@
       - publish-openstack-docs-pti
       - check-requirements
       - lib-forward-testing
+      - lib-forward-testing-python3
       - release-notes-jobs-python3
     check:
       jobs: