Fixed #26601 -- Improved middleware per DEP 0005.

Thanks Tim Graham for polishing the patch, updating the tests, and
writing documentation. Thanks Carl Meyer for shepherding the DEP.

tests/urlpatterns_reverse/middleware.py

@@ -1,37 +1,38 @@
 from django.http import HttpResponse, StreamingHttpResponse
 from django.urls import reverse
+from django.utils.deprecation import MiddlewareMixin
 
 from . import urlconf_inner
 
 
-class ChangeURLconfMiddleware(object):
+class ChangeURLconfMiddleware(MiddlewareMixin):
     def process_request(self, request):
         request.urlconf = urlconf_inner.__name__
 
 
-class NullChangeURLconfMiddleware(object):
+class NullChangeURLconfMiddleware(MiddlewareMixin):
     def process_request(self, request):
         request.urlconf = None
 
 
-class ReverseInnerInResponseMiddleware(object):
+class ReverseInnerInResponseMiddleware(MiddlewareMixin):
     def process_response(self, *args, **kwargs):
         return HttpResponse(reverse('inner'))
 
 
-class ReverseOuterInResponseMiddleware(object):
+class ReverseOuterInResponseMiddleware(MiddlewareMixin):
     def process_response(self, *args, **kwargs):
         return HttpResponse(reverse('outer'))
 
 
-class ReverseInnerInStreaming(object):
+class ReverseInnerInStreaming(MiddlewareMixin):
     def process_view(self, *args, **kwargs):
         def stream():
             yield reverse('inner')
         return StreamingHttpResponse(stream())
 
 
-class ReverseOuterInStreaming(object):
+class ReverseOuterInStreaming(MiddlewareMixin):
     def process_view(self, *args, **kwargs):
         def stream():
             yield reverse('outer')