test: only test on python 3.5, 3.9

Author: juvenn

.github/workflows/pythonpackage.yml

@@ -43,7 +43,7 @@ jobs:
     strategy:
       max-parallel: 1
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
+        python-version: [3.5, 3.8, 3.9]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python