Update python testing as per zed cycle testing runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Removing the py36 centos8 job as well as
updating the python classifier also to reflect the same.
Also indicates that we support python 3.9.

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

Change-Id: I36180f40cb2858aa051366af9b3c3c28d635632c