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 and tox.ini.

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

Change-Id: I4be58a69548a037d1920183fd1d58f91ec30e382

.zuul.yaml

@@ -2,5 +2,5 @@
     templates:
       - publish-openstack-docs-pti
       - check-requirements
-      - openstack-python3-ussuri-jobs
+      - openstack-python3-yoga-jobs
       - release-notes-jobs-python3

setup.cfg

@@ -17,6 +17,8 @@ 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
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython
 

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.18.0
-envlist = py37,pep8
+envlist = py39,pep8
 ignore_basepython_conflict = true
 
 [testenv]