Make pathlib extensible

[brettcannon]

@eorojas

and that its str produces a path with backslashes

FYI be very careful about taking the string representation of a pathlib.Path object and using it as an argument to something; you don’t want None to be a valid path. os.fspath() and os.fsdecode() both exist to get the string representation of a path-like object in the proper format. You can also use pathlib.PurePath.as_posix() to get a path with forward slashes.