Python 3.8 has reached end-of-life (no security fixes), so we no longer
test against it in CI.

- Extend GitHub Actions workflow to test against Python 3.10, 3.11, 3.12, and 3.13
- Update setup.cfg classifiers to include new Python versions

This incremental change expands test coverage across the latest Python releases,
ensuring compatibility with newer Python interpreters.

- Update GitHub Actions workflow to test against Django 4.2, 5.1, and 5.2
- Update setup.cfg classifiers to reflect new Django versions
- Modify install_requires to set minimum Django version to 4.2

This change ensures the project is tested and configured for compatibility
with the latest Django long-term support (LTS) and current releases.