middleware: use django compatibility mixin (#42266)

src/authentic2_cut/middlewares.py

@@ -1,10 +1,11 @@
 from django.conf import settings
+from django.utils.deprecation import MiddlewareMixin
 from django.utils.six.moves.urllib import parse as urlparse
 
 from authentic2.utils import same_domain, redirect
 
 
-class CUTMiddleware(object):
+class CUTMiddleware(MiddlewareMixin):
     '''Extract requesting domain from URL and set value on request and in session.
 
        Domains extracted from ?next=<url> of ?redirect_uri=<url> parameters are matched against the