Change Python Version to 3.8 in GitHub Actions

.github/workflows/unit_tests.yml

@@ -22,10 +22,10 @@ jobs:
     steps:
     - name: Checkout Code
       uses: actions/checkout@v3
-    - name: Set up Python 3.9
+    - name: Set up Python 3.8
       uses: actions/setup-python@v4
       with:
-        python-version: 3.9
+        python-version: 3.8
         cache: pip
         cache-dependency-path: setup.py
     - name: Install dependencies