Use the preview version of Poetry for the CI

.github/workflows/main.yml

@@ -49,7 +49,7 @@ jobs:
         shell: bash
         run: |
           curl -fsS -o get-poetry.py https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py
-          python get-poetry.py -y
+          python get-poetry.py -y --preview
           echo "::set-env name=PATH::$HOME/.poetry/bin:$PATH"
 
       - name: Configure poetry