Comment 4 for bug 1760400

See: https://docs.djangoproject.com/en/2.0/topics/http/middleware/#django.utils.deprecation.MiddlewareMixin

Of course there is a danger of deprecation, as with any third party and modified app we use (e.g. pybb, wiki or djangoratings). But self maintaining a small file is better than maintaining a complete app with models and views, like tracking, imho.