new_contents: from datetime import timedelta
  
  import pytest
  from rest_framework_json_api.serializers import DurationField, IntegerField
  
  from timed.serializers import PkDictSerializer
  
  
  class MyPkDictSerializer(PkDictSerializer):
      test_duration = DurationField()
      test_nr = IntegerField()
  
      class Meta:
          pk_key = 'test_nr'
          resource_name = 'my-resource'
  
  
  @pytest.fixture
  def data():
      return {
          'test_nr': 123,
          'test_duration': timedelta(hours=1),
          'invalid_field': '1234'
      }
  
  
  def test_pk_dict_serializer_single(data):
      serializer = MyPkDictSerializer(data)
  
      expected_data = {
          'test_duration': '01:00:00',
          'test_nr': 123,
      }
  
      assert expected_data == serializer.data
  
  
  def test_pk_dict_serializer_many(data):
      list_data = [
          data,
          data
      ]
      serializer = MyPkDictSerializer(list_data, many=True)
  
      expected_data = [
          {
              'test_duration': '01:00:00',
              'test_nr': 123,
          },
          {
              'test_duration': '01:00:00',
              'test_nr': 123,
          },
      ]
  
      assert expected_data == serializer.data
  
subject: Add unit test for pk dict serializer