use six.add_metaclass

Using six.add_metaclass instead of "__metaclass__" for Python 3.x
compatibility.

Change-Id: I8bf33fb1770511657587c8f7a33acfcf9a351e79
Updated from global requirements

Change-Id: I8ba2d8fe0ae0d1c4aa76207e3522f9c487d8cf9d
Merge "use six.add_metaclass"