Meta issue for in-person Jan 2020 in-person sprint

As discussed in #2587, there will be an in-person code developer sprint:

Date: Jan 6-10, 2020 Location: CFM, 23 rue de l’Université, 75007 Paris Hours: 9:30am - 6pm

The goal is to have a push towards ipywidgets 8.0.

Here are a few bullet points for items that we may want to handle for the 8.0 release

HackMD for notes

Packaging

• drop the dependency of ipywidgets to widgetsnbextension and of widgetsnbextension to ipykernel.
• pattern to split custom widget packages into a pure python package and the different extensions -> to be contributed to the cookiecutters. (Maarten wants to be in this discussion even if he cannot attend the sprints).
• should we split ipywidgets (python package) into ipycontrols and ipywidget-core?
• rewrite the slider without JQueryUI so that we can drop this dependency
• drop as much of the calls to IPython APIs. These API calls require alternative Python Jupyter kernels (such as xeus-python) to do some monkey-patching of these APIs to support ipywidgets.

New controls?

• We have PRs in for tags/colors/inputs (manipulation of sequences of the base trait types) and a text slider.

The future of BackboneJS in ipywidgets

• What alternative implementation of the observer pattern should we use?

Drop deprecated APIs

• There are a number of deprecated APIs in ipywidgets that we should finally be able to remove.

Dropping Python 2 support

• should we also take this occasion to not need the ipython_genutils package?

Drop static directory in custom widgets

• This is probably only around for legacy reasons.

Discussions on blocking / async patterns to operate on widgets

[jasongrout]

• triage the 400+ issues we have open

[SylvainCorlay]

@SylvainCorlay @jasongrout @maartenbreddels @martinRenou @vidartf @pbugnion @captainsafia @johanmabille @mariobuikhuizen @wolfv @nthiery @zerline @blois @saulshanabrook @philippjfr @nmearl @eteq @jtpio .