Add Python3 xena unit tests

This is an automatically generated patch to ensure unit testing
is in place for all the of the tested runtimes for xena.

See also the PTI in governance [1].

[1]: https://governance.openstack.org/tc/reference/project-testing-interface.html

Change-Id: Icfcaa7137318c547f27280f51268f6ca3ce8f986

zuul.d/projects.yaml

@@ -4,7 +4,7 @@
       - horizon-non-primary-django-jobs
       - horizon-nodejs10-jobs
       - openstack-lower-constraints-jobs
-      - openstack-python3-wallaby-jobs-horizon
+      - openstack-python3-xena-jobs-horizon
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     gate: