Update github actions to Python 3.8

.github/workflows/main.yml

@@ -19,10 +19,10 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - name: Setup Python 3.7
+      - name: Setup Python 3.8
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.8
 
       - name: Install dependencies
         run: |