ci: pin poetry to 1.5.1

Version 1.6.0 dropped support for Python 3.7
https://github.com/python-poetry/poetry/releases/tag/1.6.0

.github/workflows/ci.yml

@@ -39,6 +39,7 @@ jobs:
       - name: Install Poetry
         uses: snok/install-poetry@v1
         with:
+          version: '1.5.1'
           virtualenvs-in-project: true
       - name: Load cached venv
         id: cached-poetry-dependencies