Windows reserves certain filenames like 'NUL'. Checking for these names is part of a small handful of functionality that is available in pathlib but not in os.path.

I propose that we add an os.path.isreserved() function, encorporating Eryk Sun's work on bpo-27827. We then adjust pathlib to call the new function.

By doing so, we move one of the few remaining OS-specific implementations in pathlib to low-level libraries (posixpath, ntpath) where it arguably belongs.

We also make this functionality available to the segment of people using traditional string-based path operations who don't want to dip their toes into pathlib just for reserved names.