In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Moving the py36 job to py38 based as well as
updating the python classifier also to reflect the same.
[1] https://governance.openstack.org/tc/reference/runtimes/zed.html
Change-Id: I9b3a05c708d53d1e7775eefdac802392fd18bc24