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.

Note that this is effectively a revert of change
Ic93f1c77230d859713ea9bc86065880efa91de7c which attempted to resolve the
same problem in a different way.

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

Change-Id: Ibf1238bc7c09b4ae9824be81a06f22decfb40873
Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

.zuul.yaml

@@ -2,19 +2,6 @@
     templates:
       - check-requirements
       - openstack-cover-jobs
+      - openstack-python3-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
-    check:
-      jobs:
-        - openstack-tox-pep8
-        - openstack-tox-py36
-        - openstack-tox-py37
-        - openstack-tox-py38
-        - openstack-tox-py39
-    gate:
-      jobs:
-        - openstack-tox-pep8
-        - openstack-tox-py36
-        - openstack-tox-py37
-        - openstack-tox-py38
-        - openstack-tox-py39