Test against ipywidgets master

[ianhi]

Describe the issue

Now that ipywidgets master is being updated again ipympl should run a weekly workflow checking that nothing breaks. This could either be in the workflow that runs against dev matplotlib or in a separate one.

This might be as simple as:

pushd $(mktemp -d)
git clone https://github.com/jupyter-widgets/ipywidgets.git --single-branch --depth 1
cd ipywidgets
bash dev-install.sh
popd