[lesnik512]

Summary

CPython 3.15.0 is out, and modern-di 4.3.1 declares support for it. This adds 3.15 and 3.15t to both CI matrices and the 3.15 classifier.

Changes

• _checks.yml: add 3.15 and 3.15t to the pytest and floors matrices. The free-threaded interpreter check now runs on any t build, not only 3.14t.
• pyproject.toml: add the Programming Language :: Python :: 3.15 classifier.

Checklist

• Tests pass locally on 3.15.0 and 3.15.0t with the newest dependencies, at 100% coverage
• The floors install (lowest direct versions, --no-build) passes locally on 3.15.0 and 3.15.0t