update python 3.7 to 3.8

.github/workflows/ci.yml

@@ -6,10 +6,10 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v2
-      - name: Set up Python 3.7
+      - name: Set up Python 3.8
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.8
       - name: Install dependencies
         run: pip install -r requirements.txt
       - name: Run tests and collect coverage