Roll back style tests to python 3.9 (#860)

Author: hwikle-lanl

.github/workflows/unittests.yml

@@ -66,10 +66,10 @@ jobs:
     - uses: actions/checkout@v4
       with:
         submodules: recursive
-    - name: Set up Python 3.12
+    - name: Set up Python 3.9
       uses: actions/setup-python@v5
       with:
-        python-version: "3.12"
+        python-version: "3.9"
 
     - name: Install dependencies
       run: |