Update python testing classifier

Yoga testing runtime[1] has been updated to add py39
testing as voting. Unit tests update are handled by the
job template change in openstack-zuul-job

- https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/820286

this commit updates the classifier in setup.cfg file.

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

Change-Id: I1fe3704254dc33cfc1c9dc42109b1b4c7a825aef

setup.cfg

@@ -16,6 +16,8 @@ classifier =
     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 =