ci: add python 3.9 runtime

Author: abn

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         os: [Ubuntu]
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-go@v2