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.htmlml

Change-Id: I38be26a1f850275a26f54aeba40e8a809221f722

setup.cfg

@@ -18,7 +18,9 @@ classifier =
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Development Status :: 5 - Production/Stable
     Natural Language :: English
     Operating System :: POSIX :: Linux

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist =py36,py38, pep8,pylint,docs
+envlist =py39,py38, pep8,pylint,docs
 skipsdist = True
 
 [testenv]
@@ -35,8 +35,8 @@ commands = {posargs}
 commands =
   sphinx-build -W -b html doc/source doc/build/html
 
-[testenv:py36]
-basepython = python3.6
+[testenv:py39]
+basepython = python3.9
 
 [testenv:py38]
 basepython = python3.8