Add somewhat correct type hints

import itertools

from typing import TypeVar, Callable

from collections.abc import Iterable, Iterator

T = TypeVar("T")

S = TypeVar("S")

def group_by(iterable, key):

def group_by(iterable: Iterable[T], key: Callable[[T], S]) -> Iterator[tuple[S, Iterator[T]]]:

    return itertools.groupby(sorted(iterable, key=key), key)