Prepare version 2.15.0

[Kludex]

Prepare 2.15.0 for httpx2 and httpcore2 with a changelog entry for #1274.

This is a minor release because it drops support for Python 3.10, matching how #208 dropped Python 3.9 in 2.1.0.

Package versions remain derived from the release tag. No tag or release is created by this PR.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-19T19:55:57.728194Z	50e9c4c	PR opened

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[codspeed]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing prepare-2.15.0 (50e9c4c) with main (5b26c25)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩