add lib-forward-testing-python3 test job
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: Iade07e45f927de7e37ec75f09d1fda1359198470
Story: #2002586
Task: #24322

openstack/osprofiler,openstack/osprofiler,openstack/osprofiler,stackforge/osprofiler,stackforge/osprofiler,stackforge/osprofiler