Improving the type signature of `setattr`

[not-my-profile]

When the first argument of setattr is of type NoneType or EllipsisType the function will always raise an AttributeError on runtime, so it would be nice if the following calls would be detected as type errors.

x = None
setattr(x, 'foo', True) # AttributeError: 'NoneType' object has no attribute 'test'
y = ...
setattr(y, 'foo', True) # AttributeError: 'ellipsis' object has no attribute 'test'

I am not sure what's the best way to go about this is, there are probably many builtin types that do not support arbitrary attributes to be set. We do not appear to have a type to categorize such types, which could then in the future be used with Not[], once python/typing#801 is implemented.

[srittau]

The same is true when using an instance of a class that defines __slots__. I think the best we can do here is to use Any instead of object. Functionally it's mostly identical, but it signals that not all objects are valid here.

[JelleZijlstra]

With TypingError (python/typing#1043) we could perhaps special-case None. It won't cover all cases where setattr doesn't work, but doing setattr on an object of an optional type is a plausible mistake.