pythonGH-113528: Deoptimise pathlib._abc.PurePathBase.relative_to() (python#113529)

Replace use of `_from_parsed_parts()` with `with_segments()` in
`PurePathBase.relative_to()`, and move the assignment of `_drv`, `_root`
and `_tail_cached` slots into `PurePath.relative_to()`.

Author: barneygale

Lib/pathlib/__init__.py

@@ -245,7 +245,10 @@ def relative_to(self, other, /, *_deprecated, walk_up=False):
                    "scheduled for removal in Python 3.14")
             warnings.warn(msg, DeprecationWarning, stacklevel=2)
             other = self.with_segments(other, *_deprecated)
-        return _abc.PurePathBase.relative_to(self, other, walk_up=walk_up)
+        path = _abc.PurePathBase.relative_to(self, other, walk_up=walk_up)
+        path._drv = path._root = ''
+        path._tail_cached = path._raw_paths.copy()
+        return path
 
     def is_relative_to(self, other, /, *_deprecated):
         """Return True if the path is relative to another path or False.

Lib/pathlib/_abc.py

@@ -371,7 +371,7 @@ def relative_to(self, other, *, walk_up=False):
         else:
             raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")
         parts = ['..'] * step + self._tail[len(path._tail):]
-        return self._from_parsed_parts('', '', parts)
+        return self.with_segments(*parts)
 
     def is_relative_to(self, other):
         """Return True if the path is relative to another path or False.