No indication of supported python versions

[asjohnston-asf]

setup.py and setup.cfg don't provide any indication of which python version(s) are officially supported for rain-api-core. The only related information I'm able to find is that the test.yml github action uses Python 3.9 when running the test suite, but Python 3.9's end of life was Oct 31, 2025.

I propose updating setup.py to include the list of supported python versions, and expanding test.yml to run the unit tests across all those python versions.