Test one Python version on PRs, matrix nightly

[qgallouedec]

Every PR ran the test suite on 3.10, 3.11, 3.12, 3.13 and 3.14 and historically this caught almost no version-specific failures. TRL is pure Python; the version-sensitive work happens in torch/transformers.

• PRs now test 3.12 only (most used, per telemetry), like every other job in tests.yml.
• New nightly tests_python_versions.yml runs 3.10/3.11/3.13/3.14 on main, with Slack reporting and workflow_dispatch (handy before cutting a release).

Version-specific syntax regressions stay covered on PRs by ruff's target-version = "py310".

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Note

Low Risk
Changes are limited to GitHub Actions configuration and do not affect library runtime behavior or release artifacts.

Overview
PR CI no longer runs the main GPU test job across Python 3.10–3.14; it runs 3.12 only (job renamed to “Tests with latest dependencies”), aligning with the other jobs in tests.yml. Slack titles for several test jobs are shortened to drop redundant Python wording.

Multi-version coverage moves to a new workflow tests_python_versions.yml, which matrix-tests 3.10, 3.11, 3.13, and 3.14 on a daily cron (1:00 UTC) and workflow_dispatch, with Slack reporting on every run. 3.12 is intentionally omitted there because PR/main tests.yml already covers it.

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[bot-ci-comment]

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