run black over stubs, add checking to travis

django-stubs-generated/contrib/flatpages/sitemaps.pyi

@@ -3,6 +3,5 @@ from typing import Optional
 from django.contrib.sitemaps import Sitemap
 from django.db.models.query import QuerySet
 
-
 class FlatPageSitemap(Sitemap):
     def items(self) -> QuerySet: ...