Set python version for the Towncrier CI job

Otherwise the version might change depending on the runner.
We just use the same version as other jobs.

This removes a GitHub warning.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

.github/workflows/main.yml

@@ -167,6 +167,8 @@ jobs:
         uses: actions/checkout@v4
       - name: "➕ Setup Python"
         uses: actions/setup-python@v4
+        with:
+          python-version: '3.9'
       - name: "➕ Install towncrier"
         run: "pip install 'towncrier'"
       - name: "Generate changelog"