Enforce JupyterLab3 keyword for JupyterLab 3

[echarles]

I have read @ianhi suggesting using Trove classification https://github.com/pypa/trove-classifiers/blob/master/src/trove_classifiers/__init__.py.

As workaround, we could also enforce jupyterlab3 as keyword that would make it clear that we don't search on server side components of former jupyterlab2 extensions.

pip search jupyterlab3
quirkshop-jlab3-react (0.1.0)  - Quirkshop JupyterLab3 React

pip search jupyterlab
jupyterlab (2.2.9)                            - The JupyterLab notebook server extension.
  INSTALLED: 2.3.0a0
  LATEST:    2.2.9
jupyterlab-server (1.2.0)                     - JupyterLab Server
  INSTALLED: 1.2.0 (latest)
jupyterlab-wipp (0.3.0)                       - WIPP integratio
...

[echarles]

cc/ @jasongrout @jtpio @goanpeca

[goanpeca]

This is a great idea!

[goanpeca]

Would it make sense to have some "check" script so that documentation can include running this script (latest version available), that way we can enforce that before making a release of a new extension we check for some things, like including this keyword?

Much like check-manifest tool work, like jlab-check-extension.

[jasongrout]

It looks like we could clear the bar for adding a trove classifier, like Framework :: JupyterLab :: 3 and maybe Framework :: JupyterLab :: 2. Here's a recent addition, for example: pypa/trove-classifiers#53

Here is a comment about the requirements: pypa/trove-classifiers#24 (comment)

I think we could get at least 10 python packages for jlab plugins that would vouch for having a classifier.

[fcollonval]

Closing this one as both the keyword and the classifier are set by this template.