This new exception type is raised instead of `NotImplementedError` when
a path operation is not supported. It can be raised from `Path.readlink()`,
`symlink_to()`, `hardlink_to()`, `owner()` and `group()`. In a future
version of pathlib, it will be raised by `AbstractPath` for these methods
and others, such as `AbstractPath.mkdir()` and `unlink()`.