Run builds on Python 3.9

The json extension for SQLite on Windos is only included in 3.9+

.github/workflows/build.yml

@@ -14,10 +14,10 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.8
+      - name: Set up Python 3.9
         uses: actions/setup-python@v1
         with:
-          python-version: 3.8
+          python-version: 3.9
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip