title: (Django 3.1.6/Python 3.8.7) Using a named method in fields or fieldsets admin.py does not work?
body: <p>No matter what I try I cannot use a named method as a field name like they do in the docs.  Eventually I'll make the method retrieve a field of a ForeignKey but for now I just want to figure out what I'm doing wrong with this simplified example.</p>
  <p><a href="https://docs.djangoproject.com/en/3.2/ref/contrib/admin/#django.contrib.admin.ModelAdmin.empty_value_display" rel="nofollow noreferrer">In this example from docs.djangoproject.com</a> they are using the class method 'view_birth_date' just as I am using 'get_test' (below) and even through I'm using it in <code>fieldsets</code> instead of <code>fields</code> it should work.</p>
  <p>I've tried it with <code>fields = </code> and <code>fieldsets = </code> and I get the same result (error).  It cannot find the field 'get_test' either way.  I've also tried putting the <code>def get_test(self)</code> above the <code>fieldsets = </code> but it makes no difference.</p>
  <p>It should work the same as what is in the documentation but it does not.  What am I doing wrong?</p>
  <p><strong>This should work:</strong></p>
  <pre><code>@admin.register(Feeddata)
  class AdminFeeddata(admin.ModelAdmin):
      &quot;&quot;&quot;Feeddata admin.&quot;&quot;&quot;
  
      list_display = ['id', 'eventtime', 'feed']
      fieldsets = [
          ('Meta', {
              'fields': ['feed', 'eventtime', 'get_test']
          }),
          ('JSON Data', {
              'fields': ('json_data', )
          }),
      ]
  
      # pylint: disable=no-self-use
      def get_test(self):
          &quot;&quot;&quot;Test string.&quot;&quot;&quot;
          return 'test'
  </code></pre>
  <p><strong>But I get an error:</strong></p>
  <pre><code>FieldError at /admin/feeder/feeddata/fff1d764-52cd-499f-9ae9-b1251a806b5b/change/
  Unknown field(s) (get_test) specified for Feeddata. Check fields/fieldsets/exclude attributes of class AdminFeeddata.
  Request Method: GET
  Request URL:    http://localhost:8000/admin/feeder/feeddata/fff1d764-52cd-499f-9ae9-b1251a806b5b/change/
  Django Version: 3.1.6
  Exception Type: FieldError
  Exception Value:    
  Unknown field(s) (get_test) specified for Feeddata. Check fields/fieldsets/exclude attributes of class AdminFeeddata.
  Exception Location: /usr/local/lib/python3.8/site-packages/django/contrib/admin/options.py, line 711, in get_form
  Python Executable:  /usr/local/bin/python
  Python Version: 3.8.7
  Python Path:    
  ['/code',
   '/code',
   '/usr/local/bin',
   '/usr/local/lib/python38.zip',
   '/usr/local/lib/python3.8',
   '/usr/local/lib/python3.8/lib-dynload',
   '/usr/local/lib/python3.8/site-packages']
  Server time:    Thu, 18 Feb 2021 23:44:12 -0800
  </code></pre>