Hopefully this wasn't proposed before.

@attr.s
class C(object):
    x = attr.ib()
    y = attr.ib()

    @x.validator                                               
    def x_smaller_than_y(self, attribute, value):
        if value >= self.y:
            raise ValueError("'x' has to be smaller than 'y'!")

If you managed to guess what this does it means the syntax is intuitive enough :)

Similar syntax familiar from property.deleter and friends can also be useful for other parts, but for validators it feels like a natural fit (especially with the recent change to allow multiple validators).