chore(setup.py): add Django 3.1 to classifiers
chore(setup.py): add Django 3.1 to classifiers

+ sort imports

modlinltd/django-advanced-filters,dsanders11/django-advanced-filters,modlinltd/django-advanced-filters,modlinltd/django-advanced-filters,dsanders11/django-advanced-filters,dsanders11/django-advanced-filters,dsanders11/django-advanced-filters