Target: 0.4.0

- slugify now handles repeated punctuation consistently (already implemented).
- remove Python 3.9 support; add validation on Python 3.13.
- replace deprecated version parsing internals while retaining common output.
- internal: CI actions should remain checkout v4 and setup-python v5.
- known limitation: Unicode letters are transliterated poorly; do not claim this
  is fixed in 0.4.0.