test: for python 3.7

Author: juvenn

.github/workflows/pythonpackage.yml

@@ -25,7 +25,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: '3.6'
+        python-version: '3.9'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
@@ -43,7 +43,7 @@ jobs:
     strategy:
       max-parallel: 1
       matrix:
-        python-version: [3.6, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.9]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python