Corrected by revision 58958.
"with" is innocent. The problem is this function in abc.py which uses a
generator expression:

    def __instancecheck__(cls, instance):
        """Override for isinstance(instance, cls)."""
        return any(cls.__subclasscheck__(c)
                   for c in {instance.__class__, type(instance)})

It is called both by pdb (deep inside 'print') and the debugged code (in
PyObject_MethodCall), and this reveals the bug.