Set attributes to the Booster.

        Parameters
        ----------
        **kwargs
            The attributes to set.
            Setting a value to None deletes an attribute.

        Returns
        -------
        self : Booster
            Booster with set attributes.

def set_attr(self, **kwargs):
        """Set attributes to the Booster.

        Parameters
        ----------
        **kwargs
            The attributes to set.
            Setting a value to None deletes an attribute.

        Returns
        -------
        self : Booster
            Booster with set attributes.
        """
        for key, value in kwargs.items():
            if value is not None:
                if not isinstance(value, string_type):
                    raise ValueError("Only string values are accepted")
                self.__attr[key] = value
            else:
                self.__attr.pop(key, None)
        return self