[lesnik512]

Summary

CPython 3.15.0 is out. This adds 3.15 and 3.15t to the pytest matrix and the 3.15 classifier.

3.15 stays out of the floors matrix for now. The msgspec floor (0.20.0) has no 3.15 wheels; 0.22.0 is the first release with them. The floors job installs with --no-build, so it cannot pass on 3.15. Users on 3.15 get newer versions that install, and pytest covers those. Add 3.15 to floors when the msgspec floor next goes up past that release.

Changes

• _checks.yml: add 3.15 and 3.15t to pytest. Every check keyed on 3.14t now matches any t build.
• pyproject.toml: add the Programming Language :: Python :: 3.15 classifier.
• AGENTS.md names the 3.15t job next to the 3.14t one as the proof for the stress tests.

Checklist

• Every job with a 3.15 entry passes locally on 3.15.0 and 3.15.0t, run step by step from this _checks.yml
• The floors install fails locally on 3.15 for the reason above