restrict build to v3.6 and v3.9

Author: karanrk

.github/workflows/build.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.9]
 
     steps:
       - uses: actions/checkout@v2