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: Iee4060c3d566c312ece529f82bbd93c1a2661e78
Story: #2002586
Task: #24314

.zuul.yaml

@@ -6,6 +6,7 @@
       - openstack-python35-jobs
       - openstack-python36-jobs
       - lib-forward-testing
+      - lib-forward-testing-python3
       - release-notes-jobs-python3
     check:
       jobs: