Bump minimum Python requirement to 3.9?

[neubig]

From @yonetaniryo

maybe its minimum req is python 3.9 rather than 3.8 in pyproject.toml? typing of list[xxx] used in many places not yet supported in py3.8, and initially I failed to run the demo on py3.8 3.8 https://docs.python.org/3.8/library/typing.html 3.9 https://docs.python.org/3.9/library/typing.html

I think we should just bump the requirement to 3.9 (or maybe even 3.10), because 3.8 ended full support in 2021. 3.9 ended full support in 2022, but I think still a lot of people use it.