Fix import of ValidityRange in migration

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

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