Django 3.1.3 support

[a-olszewski]

ImportError: cannot import name 'FieldDoesNotExist' from 'django.db.models.fields'

From release notes:
The compatibility import of django.core.exceptions.FieldDoesNotExist in django.db.models.fields is removed.

[xtlc]

ImportError: cannot import name 'FieldDoesNotExist' from 'django.db.models.fields' (..\venv\lib\site-packages\django\db\models\fields\__init__.py)

I also tried with a django-3.1.2 installation.

[heidi-jakub]

Hi, I've prepared a fix for this error in #246.