Removal of support for end-of-life Python versions

[amas0]

Currently, cmdstanpy supports a minimum Python version of 3.8. Python 3.8 went end-of-life in October 2024. While it's possible to continue supporting these older versions, various dependencies like pandas, pytest, polars, etc. have all removed support for 3.8, so we need to use older package versions to maintain compatibility.

I'd like to propose adopting a similar practice where new releases drop support for Python versions as they go end-of-life. So, 3.8 support could be dropped now and 3.9 support would be dropped in October 2025.

This would also have the benefit of allowing the library to use newer features, which would clean up certain areas (some of the typing features especially have gotten cleaner since 3.8).

[WardBrian]

This seems reasonable to me. We removed 3.6 in #511 and 3.7 in #681 based on the same reasoning, I think we just missed the fact that 3.8 went EOL recently