sub_api get_score return value is a dict

This access pattern was wrong, and was causing errors in production. I've
included an additional happy-path test.

lms/djangoapps/grades/tasks.py

@@ -146,7 +146,7 @@ def _has_database_updated_with_new_score(
         if api_score is None:
             # Same case as the initial 'if' above, for submissions-specific scores
             return score_deleted
-        reported_modified_time = api_score.created_at
+        reported_modified_time = api_score['created_at']
     else:
         reported_modified_time = score.modified
 

lms/djangoapps/grades/tests/test_tasks.py

@@ -235,6 +235,18 @@ class RecalculateSubsectionGradeTest(ModuleStoreTestCase):
         )
         self._assert_retry_called(mock_retry)
 
+    @patch('lms.djangoapps.grades.tasks.recalculate_subsection_grade_v2.retry')
+    def test_retry_subsection_grade_on_update_not_complete_sub(self, mock_retry):
+        self.set_up_course()
+        with patch('lms.djangoapps.grades.tasks.sub_api.get_score') as mock_sub_score:
+            mock_sub_score.return_value = {
+                'created_at': datetime.utcnow().replace(tzinfo=pytz.UTC) - timedelta(days=1)
+            }
+            self._apply_recalculate_subsection_grade(
+                mock_score=MagicMock(module_type='openassessment')
+            )
+        self._assert_retry_called(mock_retry)
+
     @patch('lms.djangoapps.grades.tasks.recalculate_subsection_grade_v2.retry')
     def test_retry_subsection_grade_on_no_score(self, mock_retry):
         self.set_up_course()