GH-93521: Filter out __weakref__ slot if present in bases

[Bluenix2]

This PR fixes #93521 by moving the ('__weakref__',) if weakref_slot else () ternary into the iterator that filters out already present slots.

[bedevere-bot]

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[Bluenix2]

I am not sure why my commit was changed to pythonGH rather than GH by GitHub...

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[Bluenix2]

Ah sorry I requested your review through GitHub but I'll send a message to be clear. I have addressed your comments and pushed new changes which are passing checks.