Use ugettext_lazy to avoid django dev branch error

Author: boekkooi

oauth2_provider/models.py

@@ -4,7 +4,7 @@
 from django.db import models
 from django.utils import timezone
 from django.db.models import get_model
-from django.utils.translation import ugettext as _
+from django.utils.translation import ugettext_lazy as _
 from django.utils.encoding import python_2_unicode_compatible
 from django.core.exceptions import ImproperlyConfigured