gh-95196: Fixed instances of types.ClassMethodDescriptorType not being pickled correctly

[UltimateLobster]

NOTE: I want to apologize for anything that is not up to par in this PR. This is my first time writing to cpython (and writing in python's C-API in general), I would love to learn from any mistakes done here and improve for the next time. I tried my best to look at other examples and guides and derive the correct conventions and workflow from there.

Previously, instances of types.ClassMethodDescriptorType were reduced
into a wrong set of instructions. Instead of the instructions retrieving
the descriptor itself (using direct access to the object's __dict__),
the instructions retrieved the underlying classmethod (using the normal getattr)
which resulted in different object upon unpickling.
This change has replaced the implementation of __reduce__ for such
instances so it will directly access the owner's __dict__.

Since there is no publicly available builtin function (That I could
find) to get an attribute directly from the object's __dict__, and in
order for the implmentation to work with older versions of python, I
unfortunately could not avoid the usage of the "eval" builtin in the
implementation.

[cpython-cla-bot]

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[Fidget-Spinner]

Sorry for missing this. I'll try to review before the end of the week.

[UltimateLobster]

Sorry for missing this. I'll try to review before the end of the week.

Thank you :D

[Fidget-Spinner]

@serhiy-storchaka can you please help verify if this is the correct way to solve this problem?

[serhiy-storchaka]

Using eval() here looks clever, but perhaps too clever. One of drawbacks of eval() is that it is much slower than simple attribute resolution and dict lookup. Other is that it looks suspicious (even if it is completely safe in this case) and can trigger alarms.

In any case, I think that it is better to just disable pickling of types.ClassMethodDescriptorType, for compatibility with Python implemented classmethod descriptors and with C and Python implemented staticmethod descriptors, and because there are no use common cases for this (otherwise we would get such report years ago).