GH-110109: Add `pathlib._PurePathBase` (#110670)

Add private `pathlib._PurePathBase` class: a private superclass of both `PurePath` and `_PathBase`. Unlike `PurePath`, it does not define any of these special methods: `__fspath__`, `__bytes__`, `__reduce__`, `__hash__`, `__eq__`, `__lt__`, `__le__`, `__gt__`, `__ge__`. Its initializer and path joining methods accept only strings, not os.PathLike objects more broadly.

This is important for supporting *virtual paths*: user subclasses of `_PathBase` that provide access to archive files, FTP servers, etc. In these classes, the above methods should be implemented by users only as appropriate, with due consideration for the hash/equality of any backing objects, such as file objects or sockets.