always use get_model in migrations

core/mail/migrations/0003_emailattachment.py

@@ -3,7 +3,6 @@
 from django.db import migrations, models
 import django.db.models.deletion
 import files.models
-from mail.models import Email
 from mail.protocol import parse_email_body
 
 
@@ -24,6 +23,7 @@ class Migration(migrations.Migration):
     ]
 
     def generate_email_attachments(apps, schema_editor):
+        Email = apps.get_model('mail', 'Email')
         for email in Email.objects.all():
             raw = email.raw
             if raw is None or raw == '':