sage.tensor.modules.free_module_basis: Make Basis_abstract a subclass of AbstractFamily #30300
Description
... in particular provide an implementation of the keys() method.
We also add a generic method AbstractFamily.items() to complement the methods keys() and values(), mimicking the Mapping protocol. (Split out from #34340.)
CC: @mjungmath @egourgoulhon @tscrim
Component: linear algebra
Author: Matthias Koeppe
Branch/Commit: 66874a9
Reviewer: Eric Gourgoulhon
Issue created by migration from https://trac.sagemath.org/ticket/30300