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

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

this commit makes other required changes in setup.cfg metadata.

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

Change-Id: Idd8da33c954234b7c5d2d969c50da8aee4bf7904

Author: Ghanshyam Mann

setup.cfg

@@ -17,9 +17,8 @@ classifier =
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython
     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 =