Upgrade django-constance to 2.3.1.
Upgrade django-constance to 2.3.1.

https://github.com/jazzband/django-constance/blob/ce565ecd8acdb3bb443e864dfb8ceba4d81e2b5c/docs/changes.rst

pydata/conf_site,pydata/conf_site,pydata/conf_site