Use Python3 yoga unit tests

Because the Xena release is not being created for TripleO repos, test
runtime is not updated by automation tools. This change updates
the job template to use the tested runtime for Yoga.

Change-Id: Idaac0bd54c8cc05ef3b1de4c268cdf440b82c8e5

zuul.d/layout.yaml

@@ -3,7 +3,7 @@
       - check-requirements
       - openstack-cover-jobs
       - openstackclient-plugin-jobs
-      - openstack-python3-xena-jobs
+      - openstack-python3-yoga-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
       - tripleo-buildimage-jobs
@@ -18,7 +18,7 @@
             dependencies:
               - openstack-tox-pep8
               - openstack-tox-py36
-              - openstack-tox-py38
+              - openstack-tox-py39
         - tripleo-ci-centos-8-standalone:
             vars:
               enable_validation: true