GH-128520: pathlib ABCs: tweak protocol for virtual path strings (#134104)

Adjust `pathlib._os.vfspath()` so that it doesn't try `os.fsdecode()`. I
don't know that supporting `os.PathLike` arguments is a good idea, so
it's best to leave it out for now.

Lib/pathlib/__init__.py

@@ -188,9 +188,6 @@ class PurePath:
     def __repr__(self):
         return "{}({!r})".format(self.__class__.__name__, self.as_posix())
 
-    def __fspath__(self):
-        return str(self)
-
     def __bytes__(self):
         """Return the bytes representation of the path.  This is only
         recommended to use under Unix."""
@@ -259,6 +256,9 @@ class PurePath:
                                                   self._tail) or '.'
             return self._str
 
+    __fspath__ = __str__
+    __vfspath__ = __str__
+
     @classmethod
     def _format_parsed_parts(cls, drv, root, tail):
         if drv or root: