In this case I like the flags disappearing on older versions, just as they would if you built CPython on a version of Linux that didn't have the flags. The problem is that the Windows SDK always defines enum values for all Windows versions even if you are targeting an older version, as most APIs silently ignore unknown flags. Since there's no way to reliably remove the flags at build time, it'll have to be done at import time.

In Python, existence normally implies availability, so we should maintain that here, especially since this API raises errors with these flags.