revert to poetry 1.5.1 in CI for python 3.7 support

Author: bckohan

.github/workflows/test.yml

@@ -43,7 +43,7 @@ jobs:
     - name: Install Poetry
       uses: snok/install-poetry@v1
       with:
-        version: 1.8.3
+        version: 1.5.1
         virtualenvs-create: true
         virtualenvs-in-project: true
     - name: Install Basic Dependencies