Remove support for py26
Remove support for py26

Python 2.6 is not supported from Mitaka, as per Infra.
This patch removes the support and add py3 for the same.

Change-Id: I5cd5dd0297a8a7d7686fb384b97d634cbee53a9c

openstack/watcher-dashboard,openstack/watcher-dashboard,openstack/watcher-dashboard,openstack/watcher-dashboard