Update testing.yml

Author: yzhao062

.github/workflows/testing.yml

@@ -18,7 +18,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest]
-        python-version: [3.6, 3.9]
+        python-version: [3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2