fix(enums): failing doctest when AttributeError (#1267)

openfisca_core/indexed_enums/_enum_type.py

@@ -50,7 +50,7 @@ class EnumType(t.EnumType):
 
  >>> Enum.items
  Traceback (most recent call last):
- AttributeError: type object 'Enum' has no attribute 'items'
+ AttributeError: ...
 
  >>> class Housing(Enum):
  ... OWNER = "Owner"