run black over stubs, add checking to travis

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

@@ -3,18 +3,12 @@ from typing import Any, Dict, Optional
 
 from django.db import models
 
-
 class BaseSessionManager(models.Manager):
     creation_counter: int
     model: None
     name: None
     def encode(self, session_dict: Dict[str, int]) -> str: ...
-    def save(
-        self,
-        session_key: str,
-        session_dict: Dict[str, int],
-        expire_date: datetime,
-    ) -> AbstractBaseSession: ...
+    def save(self, session_key: str, session_dict: Dict[str, int], expire_date: datetime) -> AbstractBaseSession: ...
 
 class AbstractBaseSession(models.Model):
     session_key: Any = ...