The De Rham cohomology is in fact an algebra.

This happens in view of #29581, where the set of characteristic cohomology classes constitutes a sub-algebra.

CC: @tscrim @mkoeppe @egourgoulhon

Component: manifolds

Author: Michael Jung

Branch/Commit: fb9c661

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/32270