Use python 3.9 for CI

Author: SimmonsRitchie

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.9]
 
     steps:
       - uses: actions/checkout@v1