Make Future a generic, like in python2 (#449)

stdlib/3/concurrent/futures/_base.pyi

@@ -2,7 +2,7 @@
 #
 # NOTE: This dynamically typed stub was automatically generated by stubgen.
 
-from typing import Any
+from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Tuple
 from collections import namedtuple
 
 FIRST_COMPLETED = ... # type: Any
@@ -54,28 +54,30 @@ class _AcquireFutures:
     def __enter__(self): ...
     def __exit__(self, *args): ...
 
-def as_completed(fs, timeout=None): ...
-
 DoneAndNotDoneFutures = namedtuple('DoneAndNotDoneFutures', 'done not_done')
 
-def wait(fs, timeout=None, return_when=...): ...
+_T = TypeVar('_T')
 
-class Future:
-    def __init__(self): ...
-    def cancel(self): ...
-    def cancelled(self): ...
-    def running(self): ...
-    def done(self): ...
-    def add_done_callback(self, fn): ...
-    def result(self, timeout=None): ...
-    def exception(self, timeout=None): ...
-    def set_running_or_notify_cancel(self): ...
-    def set_result(self, result): ...
-    def set_exception(self, exception): ...
+class Future(Generic[_T]):
+    def __init__(self) -> None: ...
+    def cancel(self) -> bool: ...
+    def cancelled(self) -> bool: ...
+    def running(self) -> bool: ...
+    def done(self) -> bool: ...
+    def add_done_callback(self, fn: Callable[[Future], Any]) -> None: ...
+    def result(self, timeout: float = ...) -> _T: ...
+    def exception(self, timeout: float = ...) -> Exception: ...
+    def set_running_or_notify_cancel(self) -> None: ...
+    def set_result(self, result: _T) -> None: ...
+    def set_exception(self, exception: Exception) -> None: ...
 
 class Executor:
-    def submit(self, fn, *args, **kwargs): ...
-    def map(self, fn, *iterables, timeout=None, chunksize=1): ...
-    def shutdown(self, wait=True): ...
-    def __enter__(self): ...
-    def __exit__(self, exc_type, exc_val, exc_tb): ...
+    def submit(self, fn: Callable[..., _T], *args: Any, **kwargs: Any) -> Future[_T]: ...
+    def map(self, func: Callable[..., _T], *iterables: Any, timeout: float = ...) -> Iterable[_T]: ...
+    def shutdown(self, wait: bool = ...) -> None: ...
+    def __enter__(self) -> Executor: ...
+    def __exit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> bool: ...
+
+def as_completed(fs: Iterable[Future], timeout: float = ...) -> Iterator[Future]: ...
+
+def wait(fs: Iterable[Future], timeout: float = ..., return_when: str = ...) -> Tuple[Iterable[Future], Iterable[Future]]: ...