Apply six for metaclass

The way to using metaclass has changed in Python3.

Python 2.7 way:

class Foo(object):
  __metaclass__ = FooMeta

Python 3 way:

class Foo(object, metaclass=FooMeta):
  ...

The six.add_metaclass() decorator allows us to use one syntax that
works for both Python 2.7 and Python 3.

Change-Id: I08703a8b2927f45f4705fe085368d1ebd78b02fa
Closes-Bug: #1236648
Merge "Apply six for metaclass"