checking for metaclass to be Python 3.x compatible

The way to using metaclass has changed in Python3.(H236)

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: I24a00d9e59ffdd9abf4ccf3919737bdb079819eb
Related-Bug: #1236648
Merge "checking for metaclass to be Python 3.x compatible"