ci: test against python 3.8 and 3.9 (#120)

Author: faustbrian

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7]
+        python-version: [3.6, 3.7, 3.8, 3.9]
 
     steps:
       - uses: actions/checkout@v1