Change return type of Future.exception to be Optional[BaseException] (#3849)

Future.exception can return None if no exception was capture in the future.
As documented: https://docs.python.org/3/library/asyncio-future.html#asyncio.Future.exception

Author: bennoleslie

stdlib/3/asyncio/futures.pyi

@@ -51,7 +51,7 @@ class Future(Awaitable[_T], Iterable[_T]):
     def cancelled(self) -> bool: ...
     def done(self) -> bool: ...
     def result(self) -> _T: ...
-    def exception(self) -> BaseException: ...
+    def exception(self) -> Optional[BaseException]: ...
     def remove_done_callback(self: _S, fn: Callable[[_S], Any]) -> int: ...
     def set_result(self, result: _T) -> None: ...
     def set_exception(self, exception: Union[type, BaseException]) -> None: ...