mypy: Allow ChoiceParameter to set Literal Type

## Expected

```python
T = Literal["foo", "bar"]
t: T = luigi.ChoiceParameter(choices=typing.get_args(T), var_type=str)  # ok
```

## Actual

```python
T = Literal["foo", "bar"]
t: T = luigi.ChoiceParameter(choices=typing.get_args(T), var_type=str)  # mypy: error
```