GH-130614: pathlib ABCs: retain original separator in with_name() (#130990)

In `pathlib.types._JoinablePath.with_name()`, retain any alternative path
separator preceding the old name, rather stripping and replacing it with a
primary separator. As a result, this method changes _only_ the name.

Lib/pathlib/types.py

@@ -135,7 +135,9 @@ def with_name(self, name):
         split = self.parser.split
         if split(name)[0]:
             raise ValueError(f"Invalid name {name!r}")
-        return self.with_segments(split(str(self))[0], name)
+        path = str(self)
+        path = path.removesuffix(split(path)[1]) + name
+        return self.with_segments(path)
 
     def with_stem(self, stem):
         """Return a new path with the stem changed."""