Setter for **self.__labels** attribute.

        :param value: Attribute value.
        :type value: tuple

def labels(self, value):
        """
        Setter for **self.__labels** attribute.

        :param value: Attribute value.
        :type value: tuple
        """
        if value is not None:
            assert type(value) is tuple, "'{0}' attribute: '{1}' type is not 'tuple'!".format("labels", value)
            assert len(value) == 2, "'{0}' attribute: '{1}' length should be '2'!".format("labels", value)
            for index in range(len(value)):
                assert type(value[index]) is unicode, \
                    "'{0}' attribute element '{1}': '{2}' type is not 'unicode'!".format("labels", index, value)
        self.__labels = value