Updates the state of the given modifier key to 'released'.

def handle_modifier_up(self, modifier):
        """
        Updates the state of the given modifier key to 'released'.
        """
        _logger.debug("%s released", modifier)
        # Caps and num lock are handled on key down only
        if modifier not in (Key.CAPSLOCK, Key.NUMLOCK):
            self.modifiers[modifier] = False