Use Xena zuul job template

The template contains also py39 job.

Change-Id: I79652105a2e65a32638eed4fa76dc7e33af4a6c8

.zuul.yaml

@@ -1,7 +1,7 @@
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-python3-wallaby-jobs
+      - openstack-python3-xena-jobs
       - openstack-python37-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3