Allow for Python versions > 3.8

[s3alfisc]

Hi all,

I think it would be great to allow for Python versions > 3.8 =) For example, pyfixest requires Python >= 3.9, so users wouldn't be able to run both packages in the same project.

Additionally, have you considered to use one of the python package managers instead of setuptools? I have some
experience with poetry and pixi & would be more than happy to help you set this up.