ci(docs): lock at python 3.9 due to poetry bug

Author: tony

.github/workflows/publish-docs.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.x"]
+        python-version: ["3.9"]
     steps:
       - uses: actions/checkout@v1
       - name: Configure git