Updating python testing classifier as per Yoga testing runtime

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: Ie43070a644563307ce8ac2a13e52f54b00c0519c
diff --git a/setup.cfg b/setup.cfg
index 728e365..72c6235 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -21,6 +21,7 @@
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Topic :: Internet :: Name Service (DNS)
 
 [files]