Removed python 3.6 that failed the test

Author: KelvinYang0320

.github/workflows/python_build.yaml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.7, 3.8, 3.9]
 
     steps:
       - uses: actions/checkout@v1