Standardize on Python 3.9 per the user manual

Author: jketema

.github/actions/action.yml

@@ -45,9 +45,9 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Setup Python 3
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v4
         with:
-          python-version: "3.x"
+          python-version: "3.9"
 
       - name: Cache CodeQL
         id: cache-codeql