Add all python versions to bindep tox testing
Add all python versions to bindep tox testing

I think we want to be sure that bindep runs on all python versions,
add a tox test for them all.

Depends-On: https://review.opendev.org/737701
Change-Id: I460866456c39eb103e26d5d9adb99f2c268b81d4

openstack-infra/project-config,openstack-infra/project-config