feat(python): Add official Python 3.13 support and harden tests

This is a forward-looking release that officially brings omnipkg to the
bleeding edge of the Python ecosystem.

- **Python 3.13 Support:** The core download engine is now fully equipped
  to find, download, and bootstrap pre-release and stable versions of
  Python 3.13 from the  project.
- **Future-Proof by Design:** The  has been updated to
  officially declare Python 3.13 support to the PyPI ecosystem.
- **Test Hardening:** The UV switching test has been made more robust
  by correctly setting and restoring the install strategy, ensuring
  test purity.

Omnipkg is now ready for the future of Python.

Author: 1minds3t