ci: Use python 3.9 to avoid 3.10 poetry + dataclasses issue

See also: python-poetry/poetry#1413

Author: tony

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.x"]
+        python-version: ["3.9"]
         tmux-version: ["2.6", "2.7", "2.8", "3.0a", "3.1b", "3.2a", "master"]
     steps:
       - uses: actions/checkout@v1