where is it defined that sets are not "supposed" to contain mutable items?   such a requirement vastly limits the usefulness of sets.

Consider that relational database rows are mutable.  A result set containing multiple rows which each have a primary key comprises a set, hashed on primary key.  But other attributes of each row can be updated.   Surely this is not controversial ?  What Python data structure should I (and a whole bunch of Python ORMs) be using to represent mutable, relational database rows, unordered and unique on primary key, in memory ?