Recently it came to light that the classes in C and Python implementations of the io module have slightly different attributes (issue9858).  I propose the addition of a helper function in Lib/test/support.py to verify that the classes in two different implementations define the same attributes.  Then, we can add tests to use that function to verify that C and Python implementations define the same API (for the io module, but also for other modules where we have two implementations).  The script I added to issue9858 could serve as a starting point for such a function.

Since CPython's standard library is the de facto reference implementation, it's important that it define one API and not two slightly different ones. :-)