title: Why middleware mixin declared in django.utils.deprecation.py
body: <p>in path <code>django.utils.deprecation.py</code> we have some class about deprecation warning for methods.</p>
  
  <p>in that file we have a class called <code>MiddlewareMixin</code>. this class used to write middleware classes. Although not related to deprecation, Why this class wrote in this path?</p>