fix: Downgrade jupyter-client for papermill to work (#2105)

* As a temporary fix to Issue #2104, downgrade jupyter-client to a
  version less than v8.0.0 so that papermill is able to run.
  This is not a sufficient fix and an alternative to using papermill
  should be considered.

.github/workflows/notebooks.yml

@@ -29,7 +29,8 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip setuptools wheel
-        python -m pip install --upgrade .[test]
+        # FIXME: c.f. https://github.com/scikit-hep/pyhf/issues/2104
+        python -m pip install --upgrade .[test] 'jupyter-client<8.0.0'
 
     - name: List installed Python packages
       run: python -m pip list