[bug] [formatter] parenthesized single list comp var chokes formatter

The below line is legal python but chokes the formatter:

```
    return all(filepath.startswith(DAGIT_CLOUD_PATH) for (filepath) in diff_files)
```

Error:

```
error: Failed to format .buildkite/utils.py: syntax error: Expected `in` keyword between the `target` and `iter`.
```
