Use Python 3.8 for MyPy.

.github/workflows/mypy.yml

@@ -10,7 +10,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v1
         with:
-          python-version: 3.7
+          python-version: 3.8
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip