ci(tests): Add python 3.6 - 3.8

Author: tony

.github/workflows/tests.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [ '3.x' ]
+        python-version: [ '3.6', '3.7', '3.8' ]
     steps:
       - uses: actions/checkout@v1
       - name: Configure git