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/middleware_exceptions/middleware.py

@@ -1,8 +1,10 @@
 from __future__ import unicode_literals
 
 from django.http import HttpResponse
+from django.utils.deprecation import MiddlewareMixin
 
 
-class ProcessExceptionMiddleware(object):
+class ProcessExceptionMiddleware(MiddlewareMixin):
+
     def process_exception(self, request, exception):
         return HttpResponse('Exception caught')