PR has been posted, but it's incomplete (docs, news, etc.)

And unfortunately longer than I'd hoped, since we have to use GetProcAddress for these function on Windows 7 still (even if it has the required update), but since it's coming from kernel32 (which is always loaded) and these are going to be rare calls I'm not too concerned. Still, as soon as we drop Win7, it'll be nice to clean these up.

I ended up making the functions public as os.add_dll_directory and os.remove_dll_directory. The return value is using a capsule (which is needed because it's an opaque pointer that you use to remove the directory later), and honestly I don't think it matters enough to give it its own type. Given the choice between making the functions private (and requiring "import nt; nt._add_dll_directory()") vs. implementing a whole type for one opaque value, I'll make the functions private :)