> As discussed above, starting with msg309074, __deepcopy__() is being added to the Python implementation because it already exists in the C implementation.

Python implementation shouldn't copy all implementation details of the C implementation. This is cumbersome and often impossible. Both implementation should have the same behavior, and they do have.

> And additional tests have in fact uncovered further discrepancies between the Python and C implementations with regard to copying behavior.

What are these discrepancies?

In any case it is better to extend existing tests by adding missed checks than duplicate tests.