Update CI to use unversioned jobs template
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/

Change-Id: If46fe44ef822b26f1bc777cfbbe4fc2084fedae7
Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/reno