Enforce Py3 for `charm-glance`
Enforce Py3 for `charm-glance`

The charm has been converted to Py3 and the Py3 jobs should now
vote on changes.

Change-Id: Ibc96841bb3bac76c21e2cf3d5960d1a3befdbdaa

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