Pin trailets package as a temporary work around (#14339)

* Pin traitlets package, as a temporary measure

* oops

.github/workflows/build-test.yml

@@ -522,7 +522,7 @@ jobs:
         run: |
           python --version
           python -c "import sys;print(sys.executable)"
-          python -m pip install jupyter notebook ipykernel ipython nbconvert nbformat --pre
+          python -m pip install jupyter notebook ipykernel ipython nbconvert nbformat jupyterlab traitlets --pre
 
       # debugpy is not shipped, only installed for local tests.
       # In production, we get debugpy from python extension.

build/venv-test-ipywidgets8-requirements.txt

@@ -24,3 +24,4 @@ ipyleaflet
 jinja2==3.1.2 # https://github.com/microsoft/vscode-jupyter/issues/9468#issuecomment-1078468039
 matplotlib
 ipympl
+traitlets==5.9.0 # https://github.com/microsoft/vscode-jupyter/issues/14338

build/venv-test-requirements.txt

@@ -27,3 +27,4 @@ ipyleaflet
 jinja2==3.1.2 # https://github.com/microsoft/vscode-jupyter/issues/9468#issuecomment-1078468039
 matplotlib
 ipympl
+traitlets==5.9.0 # https://github.com/microsoft/vscode-jupyter/issues/14338