Fix underline for 'pty' in What's New in Python 3.14 (#122337)

Author: AA-Turner

Doc/whatsnew/3.14.rst

@@ -309,7 +309,7 @@ pathlib
   arguments are joined onto *other*.
 
 pty
-___
+---
 
 * Remove deprecated :func:`!pty.master_open` and :func:`!pty.slave_open`.
   They had previously raised a :exc:`DeprecationWarning` since Python 3.12.