title: django-picklefield changes structure after upgrade
body: <p>I'm migrating a Django app from 1.11.16 to 4.0.5 and django-picklefield will be migrated also from 2.0 to 3.1 .</p>
  <p>A django object has the following line:</p>
  <pre><code>from picklefield.fields import PickledObjectField
  
  class A(models.Model):
      ...
      log_list = PickledObjectField(default=list)
  </code></pre>
  <p>.</p>
  <p>Using the old code (1.11.16) and the new code (4.0.5) results a totally different result for the query:</p>
  <pre><code>#old
  Obj.objects.get(id=4737).log_list
  [{u'date': datetime.datetime(2022, 6, 27, 12, 54, 50, 746392),
  u'message': u'Fetching trademark search started',
  u'typ': u'info'},
  {u'date': datetime.datetime(2022, 6, 27, 12, 54, 53, 423384),
  u'message': u'Fetching trademark search finished',
  u'typ': u'info'}]
  #type == list
  
  #new
  Obj.objects.get(id=4737).log_list
  gAJdcQEofXECKFgEAAAAZGF0ZXEDY2RhdGV0aW1lCmRhdGV0aW1....
  #so it results exactly what is stored in the db.
  #type == str
  </code></pre>
  <p>With picklefield version 3.1 is there anything else I need to do / change to have the behaviour than with version == 2.0 ?
  What should I do with the new setup to get the list instead of the str?</p>