Apply six for metaclass
Apply six for metaclass

__metaclass__ cannot be used in python3.
six be used in general for python 3 compatibility.

Change-Id: I6d344b738cea7b9cda07cdc0e7e13fa73afa93b1
Closes-Bug: #1236648

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects