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:
Story:
Task:
- openstack-python-jobs
- openstack-python35-jobs
- lib-forward-testing
+ - lib-forward-testing-python3
- openstack-pypy-jobs-nonvoting
- publish-openstack-docs-pti
- periodic-stable-jobs
