Use doctest format with traceback in What's New entry

Author: mvanhorn

Doc/whatsnew/3.15.rst

@@ -425,20 +425,26 @@ Improved error messages
   method names from other languages (JavaScript, Java, Ruby, C#) and suggests
   the Python equivalent:
 
-  .. code-block:: pycon
+  .. doctest::
 
-     >>> [1, 2, 3].push(4)
+     >>> [1, 2, 3].push(4)  # doctest: +ELLIPSIS
+     Traceback (most recent call last):
+     ...
      AttributeError: 'list' object has no attribute 'push'. Did you mean '.append'?
 
-     >>> 'hello'.toUpperCase()
+     >>> 'hello'.toUpperCase()  # doctest: +ELLIPSIS
+     Traceback (most recent call last):
+     ...
      AttributeError: 'str' object has no attribute 'toUpperCase'. Did you mean '.upper'?
 
   When the Python equivalent is a language construct rather than a method,
   the hint describes the construct directly:
 
-  .. code-block:: pycon
+  .. doctest::
 
-     >>> {}.put("a", 1)
+     >>> {}.put("a", 1)  # doctest: +ELLIPSIS
+     Traceback (most recent call last):
+     ...
      AttributeError: 'dict' object has no attribute 'put'. Use dict[key] = value for item assignment
 
   (Contributed by Matt Van Horn in :gh:`146406`.)