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: If74666468cebc05e3089d0ea3fb864ffe7f35f1b
Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/openstack-doc-tools,openstack/openstack-doc-tools