Fix week period to string

Author: bonjourmauko

openfisca_core/periods/tests/test__parsers.py

@@ -17,7 +17,6 @@ def test__parse_period(arg, expected):
     assert _parsers._parse_period(arg) == expected
 
 
-
 @pytest.mark.parametrize("arg, error", [
     [None, AttributeError],
     [{}, AttributeError],