JupyterLab 3 only works on python >=3.5, so only depend on jupyterlab_widgets if python >=3.5.
JupyterLab 3 only works on python >=3.5, so only depend on jupyterlab_widgets if python >=3.5.

Fixes #3049

jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets