Set the Categorical to be ordered.

        Parameters
        ----------
        inplace : bool, default False
           Whether or not to set the ordered attribute in-place or return
           a copy of this categorical with ordered set to True.

def as_ordered(self, inplace=False):
        """
        Set the Categorical to be ordered.

        Parameters
        ----------
        inplace : bool, default False
           Whether or not to set the ordered attribute in-place or return
           a copy of this categorical with ordered set to True.
        """
        inplace = validate_bool_kwarg(inplace, 'inplace')
        return self.set_ordered(True, inplace=inplace)