Add support for Django 3.1
Add support for Django 3.1

In Django 3.1 the compatibility import of django.core.exceptions.FieldDoesNotExist in django.db.models.fields is removed.

So we'd have to update the package by replacing:
from django.db.models.fields import FieldDoesNotExist
with
from django.core.exceptions import FieldDoesNotExist
callowayproject/django-categories,callowayproject/django-categories,callowayproject/django-categories