CI: add py39 and py310 to experimental pipeline

The openstack-python3-jobs template covers py38 and py311, with the
assumption that intermediate Python releases should pass, too. Since
we already have an experimental pipeline to spot-check that tests
still pass on various Python versions, include those intermediate
releases as well.

Change-Id: I0b4e8839ec14a4ad7529a9929f1150e5bc586059

.zuul.yaml

@@ -37,10 +37,13 @@
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     experimental:
-      # on-demand pipeline used to test older (but still supported) versions of python
+      # on-demand pipeline used to test older (but still supported) versions of python,
+      # as well as intermediate releases that the openstack-python3-jobs might skip
       jobs:
         - openstack-tox-py36
         - openstack-tox-py37
+        - openstack-tox-py39
+        - openstack-tox-py310
     check:
       jobs:
         - swiftclient-functional: