ci: test python 3.9 (#168)

.github/workflows/tests.yml

@@ -32,15 +32,10 @@ jobs:
       - name: "Checkout repository"
         uses: actions/checkout@v4
 
-      - name: "Set up Python 3.9"
-        uses: actions/setup-python@v5
-        with:
-          python-version: "3.9"
-
       - name: "Install poetry"
         uses: abatilo/actions-poetry@v3
         with:
-          poetry-version: "2.0.1"
+          poetry-version: "2.1.2"
 
       - name: "Install icalendar ${{ matrix.icalendar-version }} and other dependencies"
         run: |