Updating yoga tested python versions in classifier

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 updates the tested py versions in setup.cfg classifier.

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

Change-Id: Ic25b27a075ab4637553fe0ce8524bc269e6b96ef

.zuul.yaml

@@ -25,7 +25,7 @@
     vars:
       rtd_webhook_id: '44747'
     templates:
-      - openstack-python3-xena-jobs
+      - openstack-python3-yoga-jobs
       - docs-on-readthedocs
       - check-requirements
     check:

setup.cfg

@@ -15,9 +15,8 @@ classifier =
     Programming Language :: Python
     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 =