use setup-python@v3 action in documentation workflow

For some reason, we use setup-python@v3 in all workflows, except the documentation one, which still had v2.

{{cookiecutter.directory_name}}/.github/workflows/documentation.yml

@@ -17,7 +17,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python 3.9
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v3
         with:
           python-version: 3.9
       - name: Python info