Start the timeout clock, used during a connect() attempt

        :raises urllib3.exceptions.TimeoutStateError: if you attempt
            to start a timer that has been started already.

def start_connect(self):
        """ Start the timeout clock, used during a connect() attempt

        :raises urllib3.exceptions.TimeoutStateError: if you attempt
            to start a timer that has been started already.
        """
        if self._start_connect is not None:
            raise TimeoutStateError("Timeout timer has already been started.")
        self._start_connect = current_time()
        return self._start_connect