Fix FieldDoesNotExist error for Django >= 3.1.2

[chrischambers]

trafficstars

In Django 3.1.2, the FieldDoesNotExist exception is not available in the django.db.models namespace, which causes errors to be thrown when adding a related model via the admin site, for example. This simple fix resolves that issue.

[chrischambers]

Fixes #123 . The linting error seems spurious.

[jevhen-ponomarenko]

Are you planning to merge it into master soon? Can I be of any help?

[OskarPersson]

@chrischambers Please fix lint errors

[MurphyAdam]

I wanted to fix the linting issues, but the logs expired and cannot see what the errors were.

[OskarPersson]

Replaced by #147