setup.py: Avoid broken polars version
This issue turned out to not be fixed in 1.7.1, so stay on 1.6.* releases: https://github.com/pola-rs/polars/issues/18719
This issue turned out to not be fixed in 1.7.1, so stay on 1.6.* releases: https://github.com/pola-rs/polars/issues/18719