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

Detected and fixed by Serhiy.

diff --cc Lib/pathlib.py
Simple merge

diff --cc Lib/test/test_pathlib.py
Simple merge

diff --cc Misc/NEWS
index 9051e00b357e77212206e2a8fc70faf67b052baf,ca4685c9f6a4f539d5b90393083bf1ec3da21f8e..d44015146734a51cba0e55201f8e49bd32808463
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -13,21 -13,9 +13,24 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #23146: Fix mishandling of absolute Windows paths with forward
+   slashes in pathlib.
+ 
 +- Issue #23096: Pickle representation of floats with protocol 0 now is the same
 +  for both Python and C implementations.
 +
 +- Issue #19105: pprint now more efficiently uses free space at the right.
 +
 +- Issue #14910: Add allow_abbrev parameter to argparse.ArgumentParser. Patch by
 +  Jonathan Paugh, Steven Bethard, paul j3 and Daniel Eriksson.
 +
 +- Issue #21717: tarfile.open() now supports 'x' (exclusive creation) mode.
 +
 +- Issue #23344: marshal.dumps() is now 20-25% faster on average.
 +
 +- Issue #20416: marshal.dumps() with protocols 3 and 4 is now 40-50% faster on
 +  average.
 +
  - Issue #23421: Fixed compression in tarfile CLI.  Patch by wdv4758h.
  
  - Issue #23361: Fix possible overflow in Windows subprocess creation code.