Allow on_setattr=setters.NO_OP on frozen classes

[bysiber]

Summary

Using on_setattr=setters.NO_OP on individual fields of a frozen class raises ValueError: Frozen classes can't use on_setattr, even though NO_OP explicitly means "do nothing for this attribute's setattr hook."

@attr.s(frozen=True)
class Frozen:
    x = attr.ib(on_setattr=setters.NO_OP)
# ValueError: Frozen classes can't use on_setattr.

This happens because the check if a.on_setattr is not None treats NO_OP the same as any real hook — NO_OP is an object() sentinel, not None, so it enters the frozen rejection branch.

The fix skips the rejection when on_setattr is specifically NO_OP, since it's semantically equivalent to having no hook at all. Real hooks like setters.validate are still correctly rejected on frozen classes.