Convert metaclasses using @six.add_metaclass()

Review Request #4923 — Created Nov. 8, 2013 and submitted — Latest diff uploaded

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master

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Convert metaclasses using @six.add_metaclass()

Metaclasses work differently in Python 2 and 3. Newer versions of six include a
decorator that can apply them for both versions. This change converts the
reviewboard codebase to use that.

Ran unit tests.