docs: Make admonition for CPy-difference use "attention" class.

This renders it in yellow/orange box on RTD server.

docs/library/sys.rst

@@ -18,6 +18,7 @@ Functions
    ``sys.stdout`` by default).
 
    .. admonition:: Difference to CPython
+      :class: attention
 
       This function appears in the ``traceback`` module in CPython.