Fix assigning-non-slot false positive with setattr

[jakelishman]

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• Add a ChangeLog entry describing what your PR does.
• [N/A] If it's a new feature, or an important bug fix, add a What's New entry in
  doc/whatsnew/<current release.rst>.
• Write a good description on what the PR does.

Type of Changes

	Type
✓	🐛 Bug fix

Description

Previously, if a class was slotted and overrode __setattr__,
assigning-non-slot would be issued when assigning to attributes. With
__setattr__ defined, we cannot infer if it is an error to assign to an
attribute, so we suppress the error.

Fix #3793

I came across that same behaviour in Qiskit/qiskit#7347, and found the old issue #3793 describing it. That's marked "high effort" whereas this is a very simple patch, so apologies if I've missed some complicating circumstances here.

[coveralls]

Pull Request Test Coverage Report for Build 1583001355

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0008%) to 93.65%

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Totals
Change from base Build 1582953346:	0.0008%
Covered Lines:	14232
Relevant Lines:	15197

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💛 - Coveralls

[jakelishman]

I've fixed the merge conflict in CONTRIBUTORS.rst, and moved the change log entry I added into the 2.13.0 release section, to match the label applied to this PR.

[Pierre-Sassoulas]

Thank you ! There will be a lot of conflicts with the first other MR being merged in main (there's only one place to put the changelog right now so conflicts are inevitable), please be mindful of your time and wait for a first review before rebasing again 😄 (also it helps with saving runner so other MR get checked faster)

[jakelishman]

Ah sure, no problem!

[Pierre-Sassoulas]

It look like the fix was indeed simpler than anticipated at first. Thank you for the very straightforward fix ! Small change-set, well tested, and a proper changelog 👌

[DanielNoord]

Small change to fix the changelog. Rest looks good to me, thanks!

[DanielNoord]

Thanks for the quick turnaround @jakelishman!

[Pierre-Sassoulas]

Congratulation on becoming a contributor too 😉

[jakelishman]

Thanks! I've used pylint a lot through general development, so it's nice I finally had a chance to give something back to it, no matter how small!