test python 3.9

Author: nschloe

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
     steps:
     - uses: actions/setup-python@v2
       with: