Fixed missing single quote in Github actions yml

Author: seanlaw

.github/workflows/github-actions.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6', '3.7', '3.8']
+        python-version: ['3.6', '3.7', '3.8']
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python