Remove conditions for Python < 3.9
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7474). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. Thanks ! can you run `make style` to fix code formatting ? then we can merge @lhoestq Done
This PR remove conditions for Python < 3.9.