Apply six for metaclass

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

Porting Change-Id I9fc7a59df3af29b4cc1287c40fa4e883d994a961
from oslo-incubator

Change-Id: Icdacdcf5556b6d3b8450d1350c6f62b4f5a9690b