[WIP] GH-89812: Add tarfile.TarPath

[barneygale]

• Split pathlib.Path into _AbstractPath and Path
  • Make _AbstractPath.stat(), open() and iterdir() abstract
  • Implement _AbstractPath._scandir() via iterdir()
    • Add follow_symlinks argument to is_dir() for consistency with os.DirEntry.is_dir()
    • Use context manager enter value in walk() to enable use of a contextlib.nullcontext()
  • Implement _AbstractPath.is_mount() and is_junction() using stat(), restoring implementation from before gh-86943: implement pathlib.WindowsPath.is_mount() #31458 in the former case.
• [more to come!]

• Issue: Add tarfile.TarPath #89812

[barneygale]

Closing - will log a fresh PR soon :)