PEP8 cleanup, unused import removal

mayan/apps/sources/models.py

@@ -13,7 +13,7 @@ from django.contrib.contenttypes import generic
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ValidationError
 from django.core.files import File
-from django.db import models, transaction
+from django.db import models
 from django.utils.translation import ugettext_lazy as _
 
 from model_utils.managers import InheritanceManager