remove deprecated isort option (#299)

Author: asherf

tox.ini

@@ -40,7 +40,7 @@ deps =
 commands =
     black --check django_prometheus/
     flake8 django_prometheus
-    isort --check-only -rc django_prometheus/
+    isort --check-only django_prometheus/
 
 [flake8]
 ignore = E501,W503