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: I844a78282f65110f6d9e41ddc752c7a5bc47a35e

setup.cfg

@@ -19,6 +19,7 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3 :: Only
     Topic :: System :: Distributed Computing
 

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.0
-envlist = py36,py38,pypy,pep8
+envlist = py36,py38,py39,pypy,pep8
 skipsdist = True
 
 [testenv]