Drop Python 3.9 Support

As [Python 3.9 is reaching EOL](https://devguide.python.org/versions/) in October 2025, we plan to drop support and set the new minimum Python requirement to 3.10.

This follows other relevant ecosystem projects in dropping support for Python 3.9:

https://github.com/pytorch/pytorch/issues/161167
https://github.com/huggingface/transformers/pull/41268
https://github.com/huggingface/peft/pull/2790