GitHub Actions: Run unit test on Python 3.13

* https://www.python.org/downloads/release/python-3130/
* https://pythoninsider.blogspot.com/2024/10/python-3130-final-released.html
* https://devguide.python.org/versions/

.github/workflows/unit_test.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.12"] #NOTE: min and max Python versions supported by icepyx
+        python-version: ["3.9", "3.13"] #NOTE: min and max Python versions supported by icepyx
 
     steps:
       - uses: "actions/checkout@v4"