Python void return type annotation

In python 3.x, it is common to use return type annotation of a function, such as:

def foo() -> str:
    return "bar"

What is the correct annotation for the “void” type?

I’m considering 3 options:

1. def foo() -> None:
   • not logical IMO, because None is not a type,
2. def foo() -> type(None):
   • using the best syntax I know for obtaining NoneType,
3. def foo():
   • omit explicit return type information.

Option 2. seems the most logical to me, but I’ve already seen some instances of 1.