Use of typing.Literal in standardized op-mode scripts provides arpgparse 'choices' for the CLI script and enum types for the API.
For example, adding:
```
ArgFamily = typing.Literal["inet", "inet6"]
...
def show(raw: bool,
family: ArgFamily,
...
```
to op-mode/route.py, may be used by opmode.py, respectively, schema_from_op_mode.py to provide:
```
root@vyos:/usr/libexec/vyos/op_mode# ./route.py show -h
usage: route.py show [-h] [--raw] --family {inet,inet6} ...
```
respectively:
```
enum FamilyRoute {
inet
inet6
}
input ShowRouteInput {
key: String!
family: FamilyRoute!
...
```