None is accepted as a type but it behaves erratically

[JukkaL]

The following fragment generates a confusing error message:

def f(x: None) -> None:
    x.f()   # Function does not return a value

[ddfisher]

After we add Optional checking, maybe we can make None more consistently mean "the type containing just the value None" instead of also "the uninhabited type" and "a value you can't use".

[ddfisher]

Fixed by #3024.