Use python3 for documentation builds
Use python3 for documentation builds

Pecanwsme 0.9 supports python3, so there is no reason to use python2
anymore.

Change-Id: Ib69afa4968af4db7a7e70e21acdb9d636694b306

stackforge/cloudkitty,openstack/cloudkitty,stackforge/cloudkitty,openstack/cloudkitty