Turns out that an AssertionError is inapropriate to raise when immutable elements are tried to be changed (see here):

exception AssertionError
Raised when an assert statement fails.

since no assert statements are involved. A better choice would be a ValueError.

CC: @egourgoulhon @tscrim @mkoeppe

Component: manifolds

Author: Michael Jung

Branch/Commit: a20a6d2

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/30275