What I'm suggesting it would be worth adding to the source code is a simple comment before the class definition for OrderedDict that says "This Python code may be overridden by an accelerated version of this class."  The idea being to prevent confusion if someone is exploring the source code for learning purposes but discovers it doesn't match what python actually does.

If you read all the way through the class and realize that the import after it does the override, you don't need the comment...but a learner exploring the source code is exactly the kind of person who might miss that, either by doing experiments before reading through the whole class or missing the implication of the import.