add cPython 3.9 testing

Author: chrissimpkins

.github/workflows/py-ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8, pypy3]
+        python-version: [3.6, 3.7, 3.8, 3.9, pypy3]
 
     steps:
       - uses: actions/checkout@v2