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 and envlist in
tox.ini.

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

Change-Id: Ic732cbd41691b11ae500eb9465423bb1d57c9a1e

setup.cfg

@@ -20,6 +20,8 @@ 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 =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py38,py36,pep8,docs
+envlist = py39,py38,py36,pep8,docs
 minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True