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: If01a4cb7dcb48698cea934309de36268ae92b7e1

setup.cfg

@@ -21,6 +21,7 @@ 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
     Programming Language :: Python :: Implementation :: CPython
 

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pypy,py38,pep8
+envlist = pypy,py39,pep8
 ignore_basepython_conflict = true
 minversion = 3.1.0
 skipsdist = True