Handle __pydantic_extra__ annotation being a string or inherited

[alexmojaki]

Change Summary

Evaluates __pydantic_extra__ to an actual type when it's a string, e.g. because of from __future__ import annotations.

Also fixes another bug in that area: the loop for tp in (cls, *cls.__mro__): didn't use the variable tp so the annotation was not checked on parent classes.

Related issue number

Closes #8647

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

[codspeed-hq]

CodSpeed Performance Report

Merging #8659 will not alter performance

_{Comparing alex/extra-annotation-str (edad102) with main (b785d5b)}

Summary

✅ 10 untouched benchmarks