This fixes the Constant class in Python 3.3.  This is sort of a replay of #603 in that, because Quantity does not define a __new__ method super().__new__ in Constant.__new__ resolves directly to object.__new__() which is now strict in taking no arguments (would have caught this sooner using 'python -Wd').  This fix is fine because Constant.__init__ also bypasses Quantity.__init__ so there's no need to pass any of the __init__ arguments to the super()'s __new__ call.