Mypy complains about the `all_equal` recipe from the itertools docs

The following function is given as a recipe in the itertools docs, indicating that it's an idiomatic usage of groupby:

from itertools import groupby

def all_equal(iterable):
    g = groupby(iterable)
    return next(g, True) and not next(g, False)

I think the correct way of adding type annotations to this function would be as follows, since it will work on arbitrary iterables:

from collections.abc import Iterable
from itertools import groupby

def all_equal(iterable: Iterable[object]) -> bool:
    g = groupby(iterable)
    return next(g, True) and not next(g, False)

Unfortunately, however, mypy complains about this function:

error: Argument 1 to "next" has incompatible type "groupby[object, object]"; expected "SupportsNext[bool]"  [arg-type]

(Mypy gives a similar error if I use Iterable[Any] instead of Iterable[object] for the argument annotation.)

Perhaps we should consider copy-and-pasting all the itertools recipes into our test_cases directory. They're all meant to be idiomatic uses of itertools, so if any of them fail to type check, there's probably a problem somewhere.

Inizia riproducendo la ricetta all_equal della documentazione di itertools con l'annotazione Iterable proposta e il comportamento attuale di mypy. Esamina la directory test_cases esistente e valuta la possibilità di aggiungervi le ricette di itertools, usando il controllo dei tipi riuscito delle ricette come criterio di completamento.