Support django 6.0

[Swamii]

• Updated test support matrix in tox.ini:
  • Dropped Python 3.8/3.9
  • Added Python 3.13/3.14
  • Moved Django targets from 3.2/4.0/4.1/4.2 to 4.2/5.2/6.0
  • Added setuptools<=71.1.0
  • Updated markdown constraint to markdown>=3.4,<4
• Updated setup.py test dependency:
  • Markdown <= 3.3.0 → Markdown >= 3.4, < 4
• Updated GitHub Actions test Python matrix in .github/workflows/main.yml:
  • 3.10, 3.11, 3.12, 3.13, 3.14
  • Removed 3.8/3.9
• Added Nix flake support:
  • New flake.nix (dev shell + Python 3.10–3.14 + uv)
  • New flake.lock