Add `--skip-magic-trailing-comma` to formatter dev comment (#8689)
Testing the compatibility with the future stable black style, i realized
the `ruff_python_formatter` dev main was lacking the
`--skip-magic-trailing-comma` option. This does not affect `ruff
format`.
Usage:
```shell
cargo run --bin ruff_python_formatter -p ruff_python_formatter -- --skip-magic-trailing-comma --emit stdout scratch.py
```
