Turn on the pull-up resistor for the specified pin if enabled is True,
        otherwise turn off the pull-up resistor.

def pullup(self, pin, enabled):
        """Turn on the pull-up resistor for the specified pin if enabled is True,
        otherwise turn off the pull-up resistor.
        """
        self._validate_channel(pin)
        if enabled:
            self.gppu[int(pin/8)] |= 1 << (int(pin%8))
        else:
            self.gppu[int(pin/8)] &= ~(1 << (int(pin%8)))
        self._write_gppu()