Prepare nilspodlib Python 3.9 and release workflow updates

[AKuederle]

Summary

• restore and verify Python 3.9 compatibility across the library
• split CI so Ruff runs once while tests cover Python 3.9, 3.10, and 3.11
• align remaining tooling/docs with Ruff and add a repo-specific Claude release skill
• harden the local and GitHub release workflow without creating the actual 4.1.2 release yet

Verification

• /home/arne/.local/bin/uv run --python 3.9 poe ci_check
• /home/arne/.local/bin/uv run --python 3.9 pytest
• /home/arne/.local/bin/uv build

Notes

• no GitHub release or PyPI publish was created in this PR
• the temporary 4.1.2 version bump was reverted before pushing this branch