[1.7.x] Fixed #23039: Don't try to serialize unmanaged models in tests

tests/migration_test_data_persistence/models.py

@@ -3,3 +3,10 @@ from django.db import models
 
 class Book(models.Model):
     title = models.CharField(max_length=100)
+
+
+class Unmanaged(models.Model):
+    title = models.CharField(max_length=100)
+
+    class Meta:
+        managed = False