Generate the choices as required by Django models.

        Returns
        -------
        tuple

def choices(cls):
        """
        Generate the choices as required by Django models.

        Returns
        -------
        tuple

        """
        # "natural" order, aka as given when instantiating
        if not hasattr(cls, '__choices_'):
            cls.__choices_ = tuple([c.choice for c in list(cls)])
        return cls.__choices_