Add openstack-python3-next-jobs

This allows us to test newer python versions. For 2027.1 the template
adds voting py315 unit tests.

Change-Id: Iee5f41a3dc22076caf9ae550bc34a17060b9a21c
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

.zuul.yaml

@@ -31,6 +31,7 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-python3-jobs
+      - openstack-python3-next-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     experimental: