fix python versions in ci

Author: sgmdlt

.github/workflows/pyci.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.8, 3.9]
     steps:
       - uses: actions/checkout@v2