MAINT: SPEC0 test env, clean up python 3.6

[jklenzing]

Description

• Cleans up code related to python 3.6 compliance now that this is no longer needed for the ops environment.
• Adds SPEC 0 tests to the main workflow, including testing against older versions of community codes
• Updates minimum scipy since 1.4 is not installable beyond python 3.8. Using minimum version from SPEC0 as base.
• Adds python 3.13 tests 🫣 (currently failing for mac) 🫣🫣🫣

Putting this up for discussion. The 2 years minimum is held to for the core scientific dependencies and cdasws. Currently pinning minimum versions for cdflib at 1.0.4 (June 2023) since this is the minimum bugfix version after the 1.0.0 release that works with all supported datasets. Also leaving pysat at 3.2.0 (Mar 2024) since this is the currently supported minimum.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Adding new minimum supported version tests in the main workflow

Test Configuration:

• Operating system: Mac, ubuntu, windows
• Version number: Python 3.9, 3.11, 3.12, 3.13

Checklist:

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

[jklenzing]

Open Questions for the draft

• Do we want to remove the minimum version for scipy? 1.4 was used because of new syntax there that was needed, the minimum python support now sort of forces us to use all versions after this.
• Do we want to maintain pysat support back two years (3.0.6)? That may create some headaches in the dev cycle, should discuss at the next tagup.
• Not sure yet whether the mac / py3.13 failure is a github actions problem or a python 3.13.0 problem. Consistently getting Fatal Python error: Bus error in that job