Return a deferred.

def cancel_subscription(self, idempotency_key=None, **params):
        """Return a deferred."""
        warnings.warn(
            'The `cancel_subscription` method is deprecated. Instead, use the '
            '`subscriptions` resource on the customer object to cancel a '
            'subscription',
            DeprecationWarning)
        url = self.instance_url() + '/subscription'
        headers = populate_headers(idempotency_key)
        d = self.request('delete', url, params, headers)
        d.addCallback(lambda response: self.refresh_from(
            {'subscription': response}, api_key, True))
        return d.addCallback(lambda _: self.subscription)