add django 3.1 compatibility: ImportError FieldDoesNotExist

[Wouter Vanden Hove]

https://docs.djangoproject.com/en/3.1/releases/3.1/#backwards-incompatible-3-1
"The compatibility import of django.core.exceptions.FieldDoesNotExist in django.db.models.fields is removed."

in dependencies.py

try:  # django>=3.1
    from django.core.exceptions import FieldDoesNotExist
except ImportError:  # django<3.1
    from django.db.models import FieldDoesNotExist