Add NotImplementedType and tighten NotImplemented type

[llchan]

We curently have

typeshed/stdlib/2and3/builtins.pyi

Line 1096 in fd57aee

NotImplemented: Any

but type(NotImplemented) is actually a built-in NotImplementedType. As far as I can tell, NotImplementedType is not exposed anywhere in PY3 at static analysis time. We should add that and also tighten the type of NotImplemented so that we can type special methods more precisely. Otherwise it's difficult to distinguish between a special method that truly returns anything and one that always returns NotImplemented.

[JelleZijlstra]

The current Any type is intentional and changing it is problematic; see Guido's comments in python/mypy#6710. As the discussion there makes clear, it's not clear that the changes proposed in this issue are going to be net positives.

[llchan]

Yeah, this was ticket was filed before the responses in the other ticket indicated that this was intentional. If you prefer to close this ticket and re-open if/when it becomes relevant again, feel free to do so.

[srittau]

I am going to close this for now. Thanks!