Fix import issue.

Author: WaVEV

django_mongodb/features.py

@@ -324,7 +324,7 @@ def django_test_expected_failures(self):
             "annotations.tests.NonAggregateAnnotationTestCase.test_empty_queryset_annotation",
             "db_functions.datetime.test_extract_trunc.DateFunctionTests.test_extract_outerref",
             "db_functions.datetime.test_extract_trunc.DateFunctionTests.test_trunc_subquery_with_parameters",
-            "expressions.tests.BasicExpressionsTests.test_aggregate_subquery_annotation"
+            "expressions.tests.BasicExpressionsTests.test_aggregate_subquery_annotation",
             "expressions.tests.BasicExpressionsTests.test_annotation_with_nested_outerref",
             "expressions.tests.BasicExpressionsTests.test_annotation_with_outerref",
             "expressions.tests.BasicExpressionsTests.test_annotations_within_subquery",