gh-93156 - fix negative indexing into absolute pathlib.PurePath().parents (GH-93273)

When a `_PathParents` object has a drive or a root, the length of the
object is *one less* than than the length of `self._parts`, which resulted
in an off-by-one error when `path.parents[-n]` was fed through to
`self._parts[:-n - 1]`. In particular, `path.parents[-1]` was a malformed
path object with spooky properties.

This is addressed by adding `len(self)` to negative indices.

Lib/pathlib.py

@@ -443,6 +443,8 @@ class _PathParents(Sequence):
 
         if idx >= len(self) or idx < -len(self):
             raise IndexError(idx)
+        if idx < 0:
+            idx += len(self)
         return self._pathcls._from_parsed_parts(self._drv, self._root,
                                                 self._parts[:-idx - 1])