Various places in the code assumed that the elements of the _supersets attribute of an open set are open, but this is not a guaranteed invariant.

We fix this by updating all loops over supersets that need to make this assumption to use a new generator function ManifoldSubset.open_supersets.

We also add the method ManifoldSubset.open_subset. It delegates to its base manifold's open_subset method, by passing a new optional parameter supersets to it.

Depends on #31674

CC: @mjungmath @egourgoulhon @tscrim

Component: manifolds

Author: Matthias Koeppe

Branch/Commit: b35eca1

Reviewer: Eric Gourgoulhon

Issue created by migration from https://trac.sagemath.org/ticket/31677