tests/cpydiff/core_class: Document issue with super in classmethod.

[AJMansfield]

Summary

This documents an existing problem with the way super() is implemented that became evident while I was implementing __init_subclass__, where when used inside a classmethod it defers to the ancestor of the class object itself (i.e. type), rather than parent classes it inherits from.

This is probably an issue with super itself not treating the second argument differently when its a type in the way that CPython does.

Testing

I've verified this against CPython versions 3.8, 3.10, and 3.12.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.42%. Comparing base (a734ee9) to head (ebbdde6).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #15509   +/-   ##
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  Coverage   98.42%   98.42%           
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  Files         161      161           
  Lines       21252    21252           
=======================================
  Hits        20918    20918           
  Misses        334      334           

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[dpgeorge]

Thanks for the test case / report.

I was curious about why the difference existed, and upon investigating managed to fix it. See #15533.