fix: drop Python 3.9 from CI matrix

Python 3.9 reached EOL in October 2025. requests>=2.33.0 requires
Python>=3.10, so we replace 3.9 with 3.10 in the test matrix.

Author: jonathannorris

.github/workflows/unit_test.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{matrix.os}}
     strategy:
       matrix:
-        python-version: ["3.12", "3.9"]
+        python-version: ["3.12", "3.10"]
         os: [ubuntu-latest, windows-latest]
 
     steps: