Improve signatures in concurrent.futures backport (#2233)

Use parameterized types in Future.add_done_callback(), wait(), and
as_completed().

Mark the traceback argument to Future.set_exception_info() as optional.

Author: li-dan

third_party/2/concurrent/futures/__init__.pyi

@@ -17,12 +17,12 @@ class Future(Generic[_T]):
     def result(self, timeout: Optional[float] = ...) -> _T: ...
     def exception(self, timeout: Optional[float] = ...) -> Any: ...
     def exception_info(self, timeout: Optional[float] = ...) -> Tuple[Any, Optional[TracebackType]]: ...
-    def add_done_callback(self, fn: Callable[[Future], Any]) -> None: ...
+    def add_done_callback(self, fn: Callable[[Future[_T]], Any]) -> None: ...
 
     def set_running_or_notify_cancel(self) -> bool: ...
     def set_result(self, result: _T) -> None: ...
     def set_exception(self, exception: Any) -> None: ...
-    def set_exception_info(self, exception: Any, traceback: TracebackType) -> None: ...
+    def set_exception_info(self, exception: Any, traceback: Optional[TracebackType]) -> None: ...
 
 class Executor:
     def submit(self, fn: Callable[..., _T], *args: Any, **kwargs: Any) -> Future[_T]: ...
@@ -37,10 +37,10 @@ class ThreadPoolExecutor(Executor):
 class ProcessPoolExecutor(Executor):
     def __init__(self, max_workers: Optional[int] = ...) -> None: ...
 
-def wait(fs: Iterable[Future], timeout: Optional[float] = ..., return_when: str = ...) -> Tuple[Set[Future], Set[Future]]: ...
+def wait(fs: Iterable[Future[_T]], timeout: Optional[float] = ..., return_when: str = ...) -> Tuple[Set[Future[_T]], Set[Future[_T]]]: ...
 
 FIRST_COMPLETED = ...  # type: str
 FIRST_EXCEPTION = ...  # type: str
 ALL_COMPLETED = ...  # type: str
 
-def as_completed(fs: Iterable[Future], timeout: Optional[float] = ...) -> Iterator[Future]: ...
+def as_completed(fs: Iterable[Future[_T]], timeout: Optional[float] = ...) -> Iterator[Future[_T]]: ...