www/py-django-sortedm2m: Update to 3.1.1

• Switch back to PyPI as it's the preferred location and the sdist is available there again.

Changelog since 3.0.2

https://github.com/jazzband/django-sortedm2m/compare/3.0.2...3.1.1

PR: 261345
Approved by: Kevin Golding (maintainer)