Issue #23146: Fix mishandling of absolute Windows paths with forward slashes in pathlib.

Detected and fixed by Serhiy.

Lib/pathlib.py

@@ -73,6 +73,10 @@ class _Flavour(object):
                     # parts. This makes the result of parsing e.g.
                     # ("C:", "/", "a") reasonably intuitive.
                     for part in it:
+                        if not part:
+                            continue
+                        if altsep:
+                            part = part.replace(altsep, sep)
                         drv = self.splitroot(part)[0]
                         if drv:
                             break