👷🐍 Enable testing on Python 3.15

[denialhaag]

Description

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Enable Python 3.15 in the Nox test matrix and package classifiers. Require NumPy 2.5.2 and scikit-learn 1.9.1 or newer on Python 3.15.

AI notice

This PR and its contents were created with the assistance of GPT-6.1 Sol via Codex.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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