As Guido pointed out on -ideas, hashing already uses the convention of `__hash__ is None` to declare a type unhashable, and `collections.abc.Hashable.__subclasshook__` already checks for that.

Meanwhile, setting `__iter__` and `__reversed__` to `None` already raises a `TypeError` on `iter` and `reversed` (although not with the most helpful description).

So, maybe that should be documented as the standard way to unimplement `__iter__` and `__reversed__`, and `collections.abc.Iterable` should check that the looked-up `__iter__` is not `None` (and presumably typecheckers doing the equivalent for both `Iterable` and `Reversible`)?