Update CI to use unversioned jobs template

As part of the migration of this project to the independent release
model, we failed to notice that the job template was still tied to a
specific release. We've now introduced a new unversioned job template,
'openstack-python3-jobs' [1], which was can and should use. Do this.

[1] https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/

Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/
Change-Id: If1b0368f69fc4b4f3040d5eb79b8d1f8a7d4c7bb
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

.zuul.yaml

@@ -3,7 +3,7 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-python3-wallaby-jobs
+      - openstack-python3-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3