GH-84783: Document GH-101264 (Make the slice object hashable) in What's New. (#102548)

Author: Yhg1s

Doc/whatsnew/3.12.rst

@@ -189,6 +189,8 @@ Other Language Changes
   part of comprehensions (like ``a``) is still disallowed, as per :pep:`572`.
   (Contributed by Nikita Sobolev in :gh:`100581`.)
 
+* :class:`slice` objects are now hashable, allowing them to be used as dict keys and
+  set items. (Contributed by Furkan Onder in :gh:`101264`.)
 
 New Modules
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