Please restore the old pattern

 if default:
    # Register the group - don't do this before adding the deps in case that step fails.
    if backend is BackendEnum.uv:
        register_default_group(group)
    elif backend is BackendEnum.none:
        # This is not really a meaningful concept without a package manager
        pass
    else:
        assert_never(backend)

The rationale being that this makes it safer in case we add a new backend, that we don't silently fail to add support for it here.

Originally posted by @nathanjmcdougall in #1589

We should explain not to remove this pattern when it is encountered, give examples of good and bad; and explain why it's positively helpful for enums in general.