PEP 255: Fix rendering of Python 2 code block (#4246)

Co-authored-by: Hugo van Kemenade <[email protected]>

Author: Viicos

peps/pep-0255.rst

@@ -5,7 +5,6 @@ Author: Neil Schemenauer <[email protected]>,
         Magnus Lie Hetland <[email protected]>
 Status: Final
 Type: Standards Track
-Content-Type: text/x-rst
 Requires: 234
 Created: 18-May-2001
 Python-Version: 2.2
@@ -289,7 +288,7 @@ otherwise appearing in ``finally`` clauses, ``except`` clauses, or in the
 Example
 =======
 
-::
+.. code-block:: python2
 
     # A binary tree class.
     class Tree: