Make the pseudocode more accurate.

pep-0553.rst

@@ -163,8 +163,9 @@ feature looks roughly like the following::
     # In builtins.
     def breakpoint(*args, **kws):
         import sys
-        hook = getattr(sys, 'breakpointhook', None)
-        if hook is None:
+        missing = object()
+        hook = getattr(sys, 'breakpointhook', missing)
+        if hook is missing:
             raise RuntimeError('lost sys.breakpointhook')
         return hook(*args, **kws)