Use object as the fallback for various Callable types

[JukkaL]

If a class with __call__ can be a subtype of Callable (#3132 implements this behavior), the fallback of Callable types should probably be object. For function objects defined using def, it's perhaps still fine to use a different callback.

Additionally, callable protocol members should probably all have a object as the fallback, even those defined using def. A concrete subtype of a protocol could implement a method using an instance of a user-defined class with __call__, which could effectively have an arbitrary fallback type.

More discussion: #3132 (comment)

cc @ilevkivskyi

[ilevkivskyi]

It looks like we could use fallbacks to relatively easily allow subclassing Callable (same as for NamedTuples and TypedDicts), see discussion in #2087. Note that this already correctly works at runtime.

[JukkaL]

I'm not convinced that this is worth the extra complexity.