* Fixed types in stdlib/3/concurrent/futures:
- Remove private classes from public interface
- Add missing types
* Remove "generated by stubgen" headers from modified type stubs
* - Use ... as default value
- Fix space formatting
* Replace more default values with '...'
* Use 'None' as default value where specified by documentation
* Use explicit Optional[T] type instead of default value None
This is necessary, because exception formatting functions like `traceback.format_exception()` expect to get `Type[BaseException]`, but sys.excepthook only provided `type`
Sebastian Meßmer