## Summary
Implement rule `mutable-fromkeys-value` (`RUF023`).
Autofixes
```python
dict.fromkeys(foo, [])
```
to
```python
{key: [] for key in foo}
```
The fix is marked as unsafe as it changes runtime behaviour. It also
uses `key` as the comprehension variable, which may not always be
desired.
Closes#4613.
## Test Plan
`cargo test`
1e4b421a00