Updating python testing as per Yoga testing runtime
Updating python testing as per Yoga testing runtime

Yoga testing runtime[1] has been updated to add py39
testing as voting and as we are testing py3.6 and py3.9
we do not need to test py3.7|8 explicitly. 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 makes update the classifier in setup.cfg file.

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

Change-Id: If47dbc692e8e2ae1eab89cd190e0299c530b9d43

openstack/os-brick,openstack/os-brick