Run CI on multiple python versions

.github/workflows/pythonapp.yml

@@ -6,13 +6,16 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
 
     steps:
-    - uses: actions/checkout@v1
-    - name: Set up Python 3.7
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v1
       with:
-        python-version: 3.7
+        python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip