Release Oscar 1.2.1

[fingolfin]

@thofma point out that we fixed at least one regression since 1.2.0 which should be quickly released. Fine by me. I think the process will be to cherry-pick it and possible more onto the release-1.2 branch in close coordination with @benlorenz. I suggested we create a list of PRs here that we'd like to see backported. Please feel free to add issues to this list, or to propose them in the comments.

We can then decide e.g. next Wednesday if the list is complete and we want to release.

• fix _as_subgroups #4277
• ...