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: I3d60f5c6ca080ac785c5355c9637788537e1e9f1
Story: #2002586
Task: #24322

.zuul.yaml

@@ -11,6 +11,7 @@
       - publish-openstack-docs-pti
       - check-requirements
       - lib-forward-testing
+      - lib-forward-testing-python3
       - release-notes-jobs-python3
       - periodic-stable-jobs
       - openstack-python36-jobs