GH-103631: Fix PurePosixPath(PureWindowsPath(...)) separator handling (GH-104949)

For backwards compatibility, accept backslashes as path separators in
`PurePosixPath` if an instance of `PureWindowsPath` is supplied.
This restores behaviour from Python 3.11.

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/pathlib.py

@@ -300,6 +300,9 @@ class PurePath(os.PathLike):
         for arg in args:
             if isinstance(arg, PurePath):
                 path = arg._raw_path
+                if arg._flavour is ntpath and self._flavour is posixpath:
+                    # GH-103631: Convert separators for backwards compatibility.
+                    path = path.replace('\\', '/')
             else:
                 try:
                     path = os.fspath(arg)