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: I6c76d04d28a610d3bf563f9cecffcdcbd432fc52
Story: #2002586
Task: #24297

.zuul.yaml

@@ -130,6 +130,7 @@
       - publish-openstack-docs-pti
       - check-requirements
       - lib-forward-testing
+      - lib-forward-testing-python3
     check:
       jobs:
         - glanceclient-dsvm-functional-v1