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: I3c746be9ddb8d6e8a310071ba2f5be54e732d356

zuul.d/layout.yaml

@@ -2,7 +2,7 @@
     templates:
       - check-requirements
       - openstack-cover-jobs
-      - openstack-python3-xena-jobs
+      - openstack-python3-yoga-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
       - tripleo-multinode-container-minimal-pipeline
@@ -26,7 +26,6 @@
               - openstack-tox-linters
               - openstack-tox-py36
               - openstack-tox-py37
-              - openstack-tox-py38
               - openstack-tox-py39
         - tripleo-buildimage-overcloud-full-centos-8:
             dependencies: *deps_unit_lint