Upgrade Jupyter Lab to version 3

[jitseniesen]

Incorporate all changes to the Notebook example in Jupyter Lab version 2 in here where relevant.

This is also the proper fix for #319.

[jitseniesen]

Jupyter Lab version 3 has just been released.

[jitseniesen]

It looks like upgrading Jupyter Lab requires upgrading qt (see this comment and further), which is not possible with Spyder 4, so I am moving this to version 0.4 of the plugin which targets Spyder 5.

[jitseniesen]

Jupyter Notebook, which I previously thought was being discontinued, is planning to release version 7 based on Jupyter Lab.

I am going to see whether this plugin can be based on Jupyter Notebook, just like it did in version 0.2.x (and before) of the plugin. This would simplify the code and make it easier to maintain the plugin, especially if we can do this without writing any JavaScript. However, I am not sure it's possible to do all we want with this approach. Indeed, I decided against this in version 0.3 of the plugin, but with the upcoming release of Jupyter Notebook v7 I want to re-evaluate this decision.