Require Python 3.12 and refresh the build

[Lawouach]

Summary

The build could no longer pass on master:

• the PDM installer used by setup-pdm does not run on Python 3.8 anymore;
• pdm.lock was stale, its hash no longer matched pyproject.toml, so pdm lock --check failed on every Python version.

This aligns the project with chaostoolkit-lib, which requires Python 3.12 since 1.45:

• requires-python = ">=3.12", classifiers for 3.12, 3.13 and 3.14;
• build matrix on 3.12, 3.13 and 3.14, actions/checkout@v7;
• pdm.lock regenerated with PDM 2.29.2, the version CI installs;
• lint rules pinned to ruff's former defaults (E4, E7, E9, F): the refreshed lock brings ruff 0.16, whose broader defaults flag 41 existing findings (pyupgrade, implicit Optional, datetime.utcnow...). Modernising the code for them can come separately;
• release workflow: Python 3.12, actions/checkout@v7, and upload-artifact/download-artifact v4 since the v2-preview versions are no longer available.

No change to the package code or its runtime dependency (chaostoolkit-lib>=1.42.1).

Testing

Ran the CI steps locally on Python 3.12, 3.13 and 3.14: pdm lock --check, pdm sync -d, pdm run lint, pdm run pytest all pass.

Unblocks #12, which will be rebased once this is merged.