Right now the .basis() method returns just a list. This should instead be an immutable Sequence, retaining the type information in its .universe() method.

CC: @sagetrac-dlucas

Component: coding theory

Author: Johan Rosenkilde, Nathann Cohen

Branch: b3dc9bf

Reviewer: David Lucas, Daniel Augot

Issue created by migration from https://trac.sagemath.org/ticket/19251