zuul: Use openstack-python3-next-jobs template

Test the project against the latest and greatest Python versions.

Change-Id: Ic1654f45dc2cc301037c7fc193485b0ec7f57fe3
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

.zuul.yaml

@@ -1,6 +1,9 @@
+---
+
 - project:
     templates:
       - openstack-python3-jobs
+      - openstack-python3-next-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - release-notes-jobs-python3