Install JupyterLab for packaging tests

Author: jasongrout

.github/workflows/packaging.yml

@@ -49,7 +49,7 @@ jobs:
 
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip wheel build
+        python -m pip install --upgrade pip wheel build jupyterlab~=3.0
     - name: Build PyPI distributions for ipywidgets
       run: |
         python -m build