Returns whether or not an asset or iterable of assets is restricted
        on a dt.

def is_restricted(self, assets, dt):
        """
        Returns whether or not an asset or iterable of assets is restricted
        on a dt.
        """
        if isinstance(assets, Asset):
            return self._is_restricted_for_asset(assets, dt)

        is_restricted = partial(self._is_restricted_for_asset, dt=dt)
        return pd.Series(
            index=pd.Index(assets),
            data=vectorize(is_restricted, otypes=[bool])(assets)
        )