Change return value of fields() from a list to a tuple.

pep-0557.rst

@@ -528,7 +528,7 @@ mutable types as default values for fields::
 Module level helper functions
 -----------------------------
 
-- ``fields(class_or_instance)``: Returns a list of ``Field`` objects
+- ``fields(class_or_instance)``: Returns a tuple of ``Field`` objects
   that define the fields for this Data Class.  Accepts either a Data
   Class, or an instance of a Data Class.  Raises `ValueError` if not
   passed a Data Class or instance of one.  Does not return