Use `_set_` as a prefix for property setters

I think there's example code that does

```python
class MyThing(lt.Thing):
    @property
    def whatever(self) -> int:
        return 42

    @whatever.set
    def set_whatever(self, val: int) -> None:
        raise NotImplementedError()
```

Adding an initial `_` would probably be better practice.