MAINT: NEP29 updates

[jklenzing]

Description

Happy Deprecation Day (observed)!

Updates the GitHub Actions main workflow to drop tests for Python 3.9. Python 3.10 is now the minimum tested version. NEP29 recommends dropping support as of Apr 5, 2024.

https://numpy.org/neps/nep-0029-deprecation_policy.html

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

via GitHub Actions

Checklist:

• [x] Make sure you are merging into the develop (not main) branch
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have linted the files updated in this pull request
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] Add a note to CHANGELOG.md, summarizing the changes
• [x] Update zenodo.json file for new code contributors

If this is a release PR, replace the first item of the above checklist with the release checklist on the wiki: https://github.com/pysat/pysat/wiki/Checklist-for-Release