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: I1d186a6759bd3e633661c75e62545d0d56700244
Story: #2002586
Task: #24320

Author: QingsZhao

.zuul.yaml

@@ -199,6 +199,7 @@
       - check-requirements
       - release-notes-jobs-python3
       - lib-forward-testing
+      - lib-forward-testing-python3
     check:
       jobs:
         - osc-functional-devstack