Install downstream packages' pre-releases

Author: davidbrochart

.github/workflows/downstream.yml

@@ -22,12 +22,12 @@ jobs:
       - name: Install dependencies
         run: |
           pip install --upgrade pip
-          pip install pyqt5 pytest \
-                      "ipykernel[test] @ git+git://github.com/ipython/ipykernel.git@master" \
-                      qtconsole[test] \
-                      "nbclient[test] @ git+git://github.com/jupyter/nbclient.git@master" \
-                      nbconvert[test] \
-                      jupyter_server[test]
+          pip install pyqt5 pytest
+          pip install ipykernel[test]       --pre
+          pip install qtconsole[test]       --pre
+          pip install nbclient[test]        --pre
+          pip install nbconvert[test]       --pre
+          pip install jupyter_server[test]  --pre
           pip install . --force-reinstall
           pip freeze
           python -c 'import jupyter_client; print("jupyter_client", jupyter_client.__version__)'