bpo-46733: move pathlib.Path.owner() and group() to PosixPath

[barneygale]

Following an idea from Alex Waygood:

Here's my two cents, as a non-expert when it comes to pathlib:

I'm not really sure why is_mount() exists on WindowsPath objects, given that it unconditionally raises NotImplementedError on WindowsPath objects -- that seems very strange to me. It seems to me like it should probably be a method on the PosixPath class rather than on the Path class.

The other methods that raise NotImplementedError don't seem as egregious, because none of them unconditionally raise NotImplementedError. We could debate whether some of them would raise different errors in an ideal world, but changing them now might have implications for backwards compatibility, and it doesn't seem like a major issue to me.

I suppose it might also be worth considering moving owner() and group() to PosixPath. In practice, these unconditionally raise NotImplementedError on Windows, since the pwd and grp modules are not available on Windows. So, in an ideal world, they probably wouldn't exist on the common base class.

Again, though, that needs to be weighed against backwards-compatibility considerations.

This makes a lot of sense to me, and lets us fix three two inappropriate uses of NotImplementedError.

https://bugs.python.org/issue46733