Drop support for python 3.8

[altheaden]

Description

Python 3.8 is no longer supported by conda-forge, so I removed references to it from setup.py.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[xylar]

@chengzhuzhang, @tomvothecoder and @forsyth2, I've tasked @altheaden with looking around at various E3SM python packages for python 3.8. It was dropped recently on conda-forge so we probably can't safely support it anymore:
conda-forge/conda-forge-pinning-feedstock#5013

[xylar]

@tomvothecoder, can you check if failing CI is something we did here or if it's a pre-existing problem?

[xylar]

It looked like it was maybe just a timeout to the LCRC server. I'll try again.

[chengzhuzhang]

@xylar @altheaden thanks for working on this!

[tomvothecoder]

Thank you for this PR @altheaden. Looks good to me!

[xylar]

Thanks @tomvothecoder and @chengzhuzhang!