Drop python3.6/3.7 support in testing runtime

In Zed cycle testing runtime, we are targetting to drop the
python 3.6/3.7 support, project started adding python 3.8 as minimum,

example nova:
- https://github.com/openstack/nova/blob/56b5aed08c6a3ed81b78dc216f0165ebfe3c3350/setup.cfg#L13

Also indicates that we support python 3.9.

Change-Id: I909c1da508e66ceafcb9f08717dad0a7cea9d50d