www/py-django-constance: Update to 3.1.0

• Switch to PyPI as the sdist is available there and it's the preferred source for Python ports.
• Add default option REDIS to reflect the settings as noted in setup.py.

Changelog since 2.9.1:

https://github.com/jazzband/django-constance/blob/3.1.0/docs/changes.rst

PR: 276432
Approved by: maintainer timeout (2+ weeks)