Run CI on Python 3.9

Author: milliams

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.8]
+        python-version: [3.8, 3.9]
     steps:
       - uses: actions/checkout@master
       - name: Set up Python