Returns the GPIO level.
        gpio:= 0-53.
        ...
        yield from pi.set_mode(23, pigpio.INPUT)
        yield from pi.set_pull_up_down(23, pigpio.PUD_DOWN)
        print(yield from pi.read(23))
        0
        yield from pi.set_pull_up_down(23, pigpio.PUD_UP)
        print(yield from pi.read(23))
        1
        ...

def read(self, gpio):
        """
        Returns the GPIO level.
        gpio:= 0-53.
        ...
        yield from pi.set_mode(23, pigpio.INPUT)
        yield from pi.set_pull_up_down(23, pigpio.PUD_DOWN)
        print(yield from pi.read(23))
        0
        yield from pi.set_pull_up_down(23, pigpio.PUD_UP)
        print(yield from pi.read(23))
        1
        ...
        """
        res = yield from self._pigpio_aio_command(_PI_CMD_READ, gpio, 0)
        return _u2i(res)