Update flake8-isort from 2.6.0 to 2.7.0 (#601)

Author: pyup-bot

requirements-development.txt

@@ -5,7 +5,7 @@ django-polymorphic==2.0.3
 factory-boy==2.11.1
 Faker==1.0.4
 flake8==3.7.7
-flake8-isort==2.6.0
+flake8-isort==2.7.0
 isort==4.3.15
 mock==2.0.0
 pytest==4.3.1