run black over stubs, add checking to travis

django-stubs-generated/contrib/sessions/serializers.pyi

@@ -3,7 +3,6 @@ from typing import Any, Dict, Optional
 from django.core.signing import JSONSerializer as BaseJSONSerializer
 from django.db.models.base import Model
 
-
 class PickleSerializer:
     def dumps(self, obj: Dict[str, Model]) -> bytes: ...
     def loads(self, data: bytes) -> Dict[str, Model]: ...