Upgrade packages to pre-release, not their test dependencies

Author: davidbrochart

.github/workflows/downstream.yml

@@ -23,9 +23,12 @@ jobs:
         run: |
           pip install --upgrade pip
           pip install pytest pytest-asyncio
-          pip install nbclient[test]        --pre
-          pip install ipyparallel[test]     --pre
-          pip install jupyter_client[test]  --pre
+          pip install nbclient[test]
+          pip install --pre -U --upgrade-strategy=only-if-needed nbclient
+          pip install ipyparallel[test]
+          pip install --pre -U --upgrade-strategy=only-if-needed ipyparallel
+          pip install jupyter_client[test]
+          pip install --pre -U --upgrade-strategy=only-if-needed jupyter_client
           pip install . --force-reinstall
           pip freeze
           python -c 'import ipykernel; print("ipykernel", ipykernel.__version__)'