gh-140490: Document versionchanged for PurePath.stem in Python 3.14

Add versionchanged note explaining that the behavior of stem changed
in Python 3.14 because a single dot is now considered a valid suffix.
Previously 'file.'.stem returned 'file.', now it returns 'file'.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kovan

Doc/library/pathlib.rst

@@ -486,6 +486,10 @@ Pure paths provide the following methods and properties:
       >>> PurePosixPath('my/library').stem
       'library'
 
+   .. versionchanged:: 3.14
+
+      A single dot ("``.``") is now considered a valid suffix.
+
 
 .. method:: PurePath.as_posix()