04·CI/CD and release automation

Why use actions/setup-python@v5 with python-version: "3.x"?

• Updates pyproject.toml with the current version.
• Only for publication — not needed for tests.
• Installs a specific Python version on the GitHub Actions runner, adds it to PATH, caches pip. Without it the runner has some random preinstalled version — not reproducible.
• Creates a virtualenv automatically.