Get or create a connection.

async def connect(self) -> Connection:
        """
        Get or create a connection.
        """
        if self._conn is None:
            self._conn = await r.connect(host=self._host,
                                         port=self._port,
                                         db=self._db,
                                         auth_key=self._auth_key,
                                         user=self._user,
                                         password=self._password,
                                         timeout=self._timeout,
                                         ssl=self._ssl,
                                         io_loop=self._loop)
        return self._conn