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: Ic90c4231bfdc2a17b525ee3a382dfa34c8c39874
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