CI: fix quoting for python-version list

Author: akx

.github/workflows/python-package.yml

@@ -17,8 +17,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [2.7,3.5,3.6,3.7, 3.8,3.9]
-
+        python-version:
+          - "2.7"
+          - "3.5"
+          - "3.6"
+          - "3.7"
+          - "3.8"
+          - "3.9"
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}