[ipspace]

Also: generalize testing documentation to explain how we choose the tested releases.

Closes #4026

[Copilot]

🟢 Approval recommended

No active issues remain, and the metadata, CI matrix, and documentation updates are aligned with the stated Python 3.15 support policy.

No open issues 🎉

What changed in this PR

This PR adds Python 3.15 to package metadata and CI/CD testing while documenting the policy of testing the oldest supported release and the two newest Python releases.

Changes:

• Adds the Python 3.15 classifier to package metadata.
• Updates push, pull-request, YAML-lint, and Ruff workflows to test Python 3.15.
• Generalizes the testing documentation and updates workflow descriptions.

File	Description
pyproject.toml	Adds the Python 3.15 classifier to the supported-version metadata.
docs/​dev/​tests.md	Documents the Python-version testing policy and updates CI workflow descriptions and table formatting.
.github/​workflows/​t-yamllint.yml	Runs YAML linting with Python 3.15.
.github/​workflows/​t-push.yml	Updates the push-test matrix to use Python 3.10, 3.14, and 3.15.
.github/​workflows/​t-pull.yml	Updates the pull-request test matrix to use Python 3.10, 3.14, and 3.15.
.github/​workflows/​ruff.yml	Runs Ruff with Python 3.15.

_{Review effort: Lite}