concurrent.futures.wait should return a NamedTuple #3380

Currently, the type of concurrent.futures.wait in typeshed is as follows:

def wait(fs: Iterable[Future[_T]], timeout: Optional[float] = ..., return_when: str = ...) -> Tuple[Set[Future[_T]],
                                                                                                    Set[Future[_T]]]: ...

According to Python's documentation:

Returns a named 2-tuple of sets. The first set, named done, contains the futures that completed (finished or cancelled futures) before the wait completed. The second set, named not_done, contains the futures that did not complete (pending or running futures).

So the type should really be:

class DoneAndNotDoneFutures(Generic[_T], NamedTuple):
    done: Set[Future[_T]]
    not_done: Set[Future[_T]]

def wait(fs: Iterable[Future[_T]], timeout: Optional[float] = ..., return_when: str = ...) -> DoneAndNotDoneFutures[_T]: ...

(the name of the NamedTuple is taken from CPython's implementation of concurrent.futures)

Unfortunately, generic NamedTuples are not currently supported by mypy, so that change would break the tests.