Model command argument as an enum

[goldseven]

    parser.add_argument(
        "command",
        choices=("set-up", "update", "shell", "internal-tests"),
        metavar="command",
        help=_("task to perform: set-up, update, or shell"),
    )

This would better be expressed using an Enum, which is type safe and supports better type hinting and auto-completion than bare string names.