Use direct model import in migration to use extensbile fields

aleksis/apps/untis/migrations/0003_guess_school_id.py

@@ -6,7 +6,8 @@ from aleksis.core.util.core_helpers import get_site_preferences
 def guess_school_id(apps, schema_editor):
     db_alias = schema_editor.connection.alias
 
-    ValidityRange = apps.get_model("chronos", "ValidityRange")
+    from aleksis.apps.chronos.models import ValidityRange
+
     try:
         vr = ValidityRange.objects.using(db_alias).first()
     except ValidityRange.DoesNotExist: