Stop serving the :attr:`.Server.sockets`.

async def close(self):
        """Stop serving the :attr:`.Server.sockets`.
        """
        if self._server:
            self._server.close()
            self._server = None
            coro = self._close_connections()
            if coro:
                await coro
            self.logger.debug('%s closed', self)
            self.event('stop').fire()