py-ipywidgets: updated to 7.6.3

7.6
---

To see the full list of pull requests and issues, see the [7.6.0 milestone](https://github.com/jupyter-widgets/ipywidgets/milestone/31?closed=1) on GitHub.

The main change in this release is that installing `ipywidgets` 7.6.0 will now automatically enable ipywidgets support in JupyterLab 3.0窶蚤 user has no extra JupyterLab installation step and no rebuild of JupyterLab, nor do they need Node.js installed. Simply install the python ipywidgets package with pip (`pip install ipywidgets==7.6.0`) or conda/mamba (`conda install -c conda-forge ipywidgets=7.6.0`) and ipywidgets will automatically work in classic Jupyter Notebook and in JupyterLab 3.0.

This is accomplished with the new python package `jupyterlab_widgets` version 1.0, on which `ipywidgets` 7.6.0 now depends (similar to how `ipywidgets` already depends on the `widgetsnbextension` package to configure ipywidgets for the classic Jupyter Notebook). The `jupyterlab_widgets` Python package is a JupyterLab 3.0 prebuilt extension, meaning that it can be installed into JupyterLab 3.0 without rebuilding JupyterLab and without needing Node.js installed.

Updates for Widget Maintainers

Custom widget maintainers will need to make two changes to update for JupyterLab 3:

1. Update the `@jupyter-widgets/base` dependency version to include `^4` to work in JupyterLab 3.0. For example, if you had a dependency on `@jupyter-widgets/base` version `^2 || ^3`, update to `^2 || ^3 || ^4` for your widget to work in classic Jupyter Notebook, JupyterLab 1, JupyterLab 2, and JupyterLab 3.
2. In the `package.json`, add the following `sharedPackages` configuration inside the `jupyterlab` key. See the [JupyterLab extension documentation](https://jupyterlab.readthedocs.io/en/stable/extension/extension_dev.html#requiring-a-service) for more information.

  ```json
    "jupyterlab": {
      "sharedPackages": {
        "@jupyter-widgets/base": {
          "bundled": false,
          "singleton": true
        }
      }
    }
  ```

Separate from these two steps to update for JupyterLab 3, we also recommend that you make your widget's JupyterLab extension a prebuilt extension for JupyterLab 3.0. Users will be able to install your JupyterLab 3.0 prebuilt extension without rebuilding JupyterLab or needing Node.js. See the [JupyterLab 3 extension developer documentation](https://jupyterlab.readthedocs.io/en/stable/extension/extension_dev.html) or the new [widget extension cookiecutter](https://github.com/jupyter-widgets/widget-ts-cookiecutter/tree/jlab3) for more details.

(adam)