Add support for instance method reassignment when extra='allow'

[sydney-runkle]

Change Summary

Adding aligning the logic of __setattr__ and __getattr__ when extra=True on a model's config.

Related issue number

Fix #7631

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @Kludex

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[sydney-runkle]

Please review

Reviews welcome - not super set on a given pattern / if we should even do this, just looking for feedback 😄

[adriangb]

What happens if the model is also frozen?

[dmontagu]

Lol good question.

I checked, and:

• On main, you get an error if you try to replace a method on a frozen model instance with or without extra='allow'.
• On this branch, you get an error if you try to replace a method on a frozen model instance with or without extra='allow'.
• For what it's worth, dataclasses raises an error if you try to overwrite a method on a frozen dataclass instance

I think this is reasonable behavior for frozen models (i.e., frozen instances don't let you overwrite any attributes, not just fields/extra/etc.). And since it raises an error, if we did ever want to change this in the future it would be easier to justify in terms of backwards compatibility than going in the other direction (i.e., making a non-error into an error).