ci: Lock python at 3.9 to avoid poetry bug

See also: python-poetry/poetry#2386

Author: tony

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.x"]
+        python-version: ["3.9"]
     steps:
       - uses: actions/checkout@v1
       - name: Configure git