nova not installable on py2 environments which breaks the gate

As noted in the revert https://review.opendev.org/#/c/694891/ change https://review.opendev.org/#/c/687954/ made nova not installable on environments running python < 3.6:

http://logstash.openstack.org/#dashboard/file/logstash.json?query=message%3A%5C%22nova%20requires%20Python%20'%3E%3D3.6'%20but%20the%20running%20Python%20is%5C%22%20AND%20tags%3A%5C%22console%5C%22&from=7d

Since devstack isn't by default using py3 lots of downstream jobs broke in other projects.

This bug is to track the fix: https://review.opendev.org/#/c/695007/