Only install jupyterlab_widgets if py3.6 or above
Only install jupyterlab_widgets if py3.6 or above

For jupyterlab_widgets 1.0.0, only Python 3.6 and above are supported, so
restrict it as part of the extra_requires.

Fixes https://github.com/jupyter-widgets/ipywidgets/issues/3058.

jupyterlab_widgets 1.0.0 documentation:
https://pypi.org/project/jupyterlab-widgets/1.0.0/

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