You can use Iterator type, for example this works in mypy (didn't test in other type checkers):

  def f() -> Iterator[int]:
      yield 42

In case you want annotate something specifically as Generator[int, None, None] (for example to use its .close() method), then you can create a generic alias:

  T = TypeVar('T')
  Gen = Generator[T, None, None]

  def f() -> Gen[int]:
      ...

this is supported by mypy as well.