I am not a Windows person... but is there a reason that handle_list must be an attribute of a STARTUPINFO class rather than just a special case of pass_fds such that people could supply windows handles in the pass_fds parameter rather than using STARTUPINFO?  (we can detect which is which based on type right?  or are they both integers and thus indistinguishable?)

The whole STARTUPINFO thing feels like we are exposing windows internals here and not offering an abstract API that people would write portable code to get the same behavior across OSes on without platform conditionals of their own.

But maybe that is required here given how little I know of Windows?  Food for thought.