ci: pytest 8 warns for deprecation warning of nose

See #3883

.github/workflows/tests.yml

@@ -96,7 +96,7 @@ jobs:
           pip install file://$PWD/python/ipywidgets#egg=ipywidgets[test]
       - name: Test with pytest
         run: |
-          pip install pytest
+          pip install "pytest<8"
           cd python/ipywidgets
           pytest --cov=ipywidgets ipywidgets