We should allow ERROR_INVALID_FUNCTION (1), ERROR_INVALID_PARAMETER (87), and ERROR_NOT_SUPPORTED (50) for readlink and _getfinalpathname, which can indicate a device that does not implement or is not mounted by a file system. We should also allow ERROR_BAD_NET_NAME (67, "the network name cannot be found"), which indicates that a server or share isn't found when opening a UNC path.

I don't know whether ERROR_INVALID_NAME (123) should be allowed. Also, it hasn't been added already, but I'd be equally unsure about adding ERROR_BAD_PATHNAME (161). These aren't like a missing file, path, or server, or an unsupported device. I know Python's realpath() is supposed to be permissive, but that's going too far I think.

Returning r"\\.\nul" is fine. I'd prefer to change os.devnull to match it. Scripts should be able to handle this since already abspath(os.devnull) is r"\\.\nul".