Add Python 3.6 to build matrix

Author: lqmanh

.github/workflows/ci-python.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9]
     steps:
       - name: Checkout
         uses: actions/checkout@v2