Define __unicode__ and __str__ methods on the given class in Python 2.

    The given class must define a __str__ method returning a unicode string,
    otherwise a TypeError is raised.
    Under Python 3, the class is returned as is.

def string_class(cls):
    """Define __unicode__ and __str__ methods on the given class in Python 2.

    The given class must define a __str__ method returning a unicode string,
    otherwise a TypeError is raised.
    Under Python 3, the class is returned as is.
    """
    if not PY3:
        if '__str__' not in cls.__dict__:
            raise TypeError('the given class has no __str__ method')
        cls.__unicode__, cls.__string__ = (
            cls.__str__, lambda self: self.__unicode__().encode('utf-8'))
    return cls