[Minor] GUI for matplotlib to run examples

[mvargas33]

If you try to run the examples on a fresh virtual environment, no figure will be shown and you will end up with the following error:

UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.

I propose to add pyqt5 to 'demo' dependencies. This is a package with a GUI that works for matplotlib. This solves the issue.

[terrytangyuan]

LGTM

[bellet]

Thanks @mvargas33. I am not quite familiar with these GUI aspects and their potential dependencies. Perhaps @perimosocordiae can validate as well

[perimosocordiae]

I don't think we need to specify QT as a dependency here, there are several matplotlib backends that should all work reasonably well and I wouldn't want to make users download the QT backend if they aren't going to use it.

Maybe the best change here is a comment in the demo noting that a graphical matplotlib backend is recommended.

[mvargas33]

I followed @perimosocordiae recommendation and added a note in the docs for sandwich and algorithms walkthrough instead. That should be enough.