Make concurrent.futures stubs identical (#2237)

Make the Python 2 and 3 concurrent.futures stubs identical so fixes get
applied to both.

For example, #1305 and #2233 fixed the same problem at different times,
as did #1078 and #1911.

By making the stubs identical, we apply the fix from #1711 to Python 2.

Fixes #2234.

stdlib/3/concurrent/futures/_base.pyi

@@ -1,4 +1,6 @@
 from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Tuple, Optional, Set, NamedTuple
+from types import TracebackType
+import sys
 
 FIRST_COMPLETED = ...  # type: str
 FIRST_EXCEPTION = ...  # type: str
@@ -24,14 +26,25 @@ class Future(Generic[_T]):
     def done(self) -> bool: ...
     def add_done_callback(self, fn: Callable[[Future[_T]], Any]) -> None: ...
     def result(self, timeout: Optional[float] = ...) -> _T: ...
-    def exception(self, timeout: Optional[float] = ...) -> Optional[BaseException]: ...
     def set_running_or_notify_cancel(self) -> bool: ...
     def set_result(self, result: _T) -> None: ...
-    def set_exception(self, exception: Optional[BaseException]) -> None: ...
+
+    if sys.version_info >= (3,):
+        def exception(self, timeout: Optional[float] = ...) -> Optional[BaseException]: ...
+        def set_exception(self, exception: Optional[BaseException]) -> None: ...
+    else:
+        def exception(self, timeout: Optional[float] = ...) -> Any: ...
+        def exception_info(self, timeout: Optional[float] = ...) -> Tuple[Any, Optional[TracebackType]]: ...
+        def set_exception(self, exception: Any) -> 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]: ...
-    def map(self, func: Callable[..., _T], *iterables: Iterable[Any], timeout: Optional[float] = ..., chunksize: int = ...) -> Iterator[_T]: ...
+    if sys.version_info >= (3, 5):
+        def map(self, func: Callable[..., _T], *iterables: Iterable[Any], timeout: Optional[float] = ..., chunksize: int = ...) -> Iterator[_T]: ...
+    else:
+        def map(self, func: Callable[..., _T], *iterables: Iterable[Any], timeout: Optional[float] = ...,) -> Iterator[_T]: ...
     def shutdown(self, wait: bool = ...) -> None: ...
     def __enter__(self: _T) -> _T: ...
     def __exit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> bool: ...

stdlib/3/concurrent/futures/process.pyi

@@ -1,9 +1,11 @@
 from typing import Optional, Any
 from ._base import Future, Executor
+import sys
 
 EXTRA_QUEUED_CALLS = ...  # type: Any
 
-class BrokenProcessPool(RuntimeError): ...
+if sys.version_info >= (3,):
+    class BrokenProcessPool(RuntimeError): ...
 
 class ProcessPoolExecutor(Executor):
     def __init__(self, max_workers: Optional[int] = ...) -> None: ...