Update middlewares to new style

https://docs.djangoproject.com/en/2.0/topics/http/middleware/#upgrading-middleware

Author: stsewd

readthedocs/core/middleware.py

@@ -1,19 +1,21 @@
 """Middleware for core app."""
 
-from __future__ import absolute_import
-from builtins import object
+from __future__ import (
+    absolute_import, division, print_function, unicode_literals)
+
 import logging
 
-from django.utils.translation import ugettext_lazy as _
 from django.conf import settings
 from django.contrib.sessions.middleware import SessionMiddleware
 from django.core.cache import cache
-from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
-from django.core.urlresolvers import set_urlconf, get_urlconf
+from django.core.exceptions import MultipleObjectsReturned, ObjectDoesNotExist
+from django.core.urlresolvers import get_urlconf, set_urlconf
 from django.http import Http404, HttpResponseBadRequest
+from django.utils.deprecation import MiddlewareMixin
+from django.utils.translation import ugettext_lazy as _
 
 from readthedocs.core.utils import cname_to_slug
-from readthedocs.projects.models import Project, Domain
+from readthedocs.projects.models import Domain, Project
 
 log = logging.getLogger(__name__)
 
@@ -30,7 +32,7 @@
 )
 
 
-class SubdomainMiddleware(object):
+class SubdomainMiddleware(MiddlewareMixin):
 
     """Middleware to display docs for non-dashboard domains."""
 
@@ -138,12 +140,12 @@ def process_response(self, request, response):
         return response
 
 
-class SingleVersionMiddleware(object):
+class SingleVersionMiddleware(MiddlewareMixin):
 
     """
     Reset urlconf for requests for 'single_version' docs.
 
-    In settings.MIDDLEWARE_CLASSES, SingleVersionMiddleware must follow after
+    In settings.MIDDLEWARE, SingleVersionMiddleware must follow after
     SubdomainMiddleware.
     """
 
@@ -192,7 +194,7 @@ def process_request(self, request):
 
 
 # Forked from old Django
-class ProxyMiddleware(object):
+class ProxyMiddleware(MiddlewareMixin):
 
     """
     Middleware that sets REMOTE_ADDR based on HTTP_X_FORWARDED_FOR, if the.

readthedocs/settings/base.py

@@ -126,7 +126,7 @@ def INSTALLED_APPS(self):  # noqa
     def USE_PROMOS(self):  # noqa
         return 'readthedocsext.donate' in self.INSTALLED_APPS
 
-    MIDDLEWARE_CLASSES = (
+    MIDDLEWARE = (
         'readthedocs.core.middleware.ProxyMiddleware',
         'readthedocs.core.middleware.FooterNoSessionMiddleware',
         'django.middleware.locale.LocaleMiddleware',