Deregisters an existing key.

            `key`
                String key to deregister.

            Returns boolean.

def deregister(self, key):
        """ Deregisters an existing key.

            `key`
                String key to deregister.

            Returns boolean.
            """

        res = super(ExtRegistry, self).deregister(key)
        if key in self._type_info:
            del self._type_info[key]
        return res