use system default python version

.github/workflows/pythonpackage.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7]
+        python-version: [3.6, 3.7, 3.8]
 
     steps:
     - uses: actions/checkout@v1
@@ -25,7 +25,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install pipenv
-        pipenv install --dev --system
+        pipenv install --dev --system  
     - name: Lint with flake8
       run: |
         make flake8