Note in migration guide the privatization of .widgets and .widget_types

[jasongrout]

Problem

The migration guide does not cover the privatization of .widgets and .widget_types, as noted here.

Suggested Improvement

Put a note in the user migration guide that covers these usecases and provides a shim for accessing the variables in ipywidgets 7 and 8 if needed (though note that the variable is now private in ipywidgets 8, so references are discouraged).

[maartenbreddels]

See also how I fix this in react-ipywidgets:
widgetti/reacton@6fb0784

[jasongrout]

In the dev meeting today, @maartenbreddels volunteered to put in a PR that deprecates (but doesn't remove) the Widget class .widgets and .widget_types attributes (i.e., reverses their removal, and makes it a deprecation). We prefer to have this as a module-level constant (for example, possibly named WIDGETS and WIDGET_TYPES), rather than as a class attribute.