Updating python testing as per Yoga testing runtime

Yoga testing runtime has been updated with py38 and py39
as voting and removed the py36 testing. Unit tests update are
handled by the job template change in openstack-zuul-job and you
can see the updated jobs running in gate.

- https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/818609

this commit makes other required changes in obs template and
update the clasifier in setup.cfg file.

[1] https://governance.openstack.org/tc/reference/runtimes/yoga.html

Change-Id: I686b17209e94779d6e29879730b635c17e338c76

setup.cfg

@@ -17,8 +17,8 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =

zuul.d/project.yaml

@@ -2,7 +2,7 @@
     templates:
       - check-requirements
       - openstack-cover-jobs
-      - openstack-python3-xena-jobs
+      - openstack-python3-yoga-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check: