Update viewers.

        DEPRECATED:  use ``policy["roles/viewers"] = value`` instead.

def viewers(self, value):
        """Update viewers.

        DEPRECATED:  use ``policy["roles/viewers"] = value`` instead.
        """
        warnings.warn(
            _ASSIGNMENT_DEPRECATED_MSG.format("viewers", VIEWER_ROLE),
            DeprecationWarning,
        )
        self[VIEWER_ROLE] = value