pythonGH-113528: Deoptimise pathlib._abc.PathBase._make_child_relpath()

Call straight through to `joinpath()` in `PathBase._make_child_relpath()`.
Move optimised/caching code to `pathlib.Path._make_child_relpath()`

Author: barneygale

Lib/pathlib/__init__.py

@@ -299,6 +299,22 @@ def iterdir(self):
     def _scandir(self):
         return os.scandir(self)
 
+    def _make_child_relpath(self, name):
+        path_str = str(self)
+        tail = self._tail
+        if tail:
+            path_str = f'{path_str}{self.pathmod.sep}{name}'
+        elif path_str != '.':
+            path_str = f'{path_str}{name}'
+        else:
+            path_str = name
+        path = self.with_segments(path_str)
+        path._str = path_str
+        path._drv = self.drive
+        path._root = self.root
+        path._tail_cached = tail + [name]
+        return path
+
     def absolute(self):
         """Return an absolute version of this path
         No normalization or symlink resolution is performed.

Lib/pathlib/_abc.py

@@ -789,20 +789,7 @@ def _scandir(self):
         return nullcontext(self.iterdir())
 
     def _make_child_relpath(self, name):
-        path_str = str(self)
-        tail = self._tail
-        if tail:
-            path_str = f'{path_str}{self.pathmod.sep}{name}'
-        elif path_str != '.':
-            path_str = f'{path_str}{name}'
-        else:
-            path_str = name
-        path = self.with_segments(path_str)
-        path._str = path_str
-        path._drv = self.drive
-        path._root = self.root
-        path._tail_cached = tail + [name]
-        return path
+        return self.joinpath(name)
 
     def glob(self, pattern, *, case_sensitive=None, follow_symlinks=None):
         """Iterate over this subtree and yield all existing files (of any