The concurrent.futures.Future class's set_exception() method might be
called with a BaseException that is not an Exception, so change
set_exception()'s parameter type from Exception to BaseException. The
exception set via set_exception() is returned by exception(), so
change exception()'s return type from Exception to BaseException.
* 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
