fix: make .widget and .widget_types deprecated

[maartenbreddels]

In #3122 we renamed .widget and .widget_types to ._active_widgets
and ._widget_types. That breaks code, and we did not have a deprecation
period.
This PR makes the dict and registry non-members of the Widget class
and puts in a backwards compatible way to deprecated these members.

Closes #3562

[github-actions]

👈 Launch a binder notebook on branch maartenbreddels/ipywidgets/fix_widgets_and_widget_types

[jasongrout]

The two conventions for module-level variables I see in pep8 are:

Global variable names:

(Let’s hope that these variables are meant for use inside one module only.) The conventions are about the same as those for functions.

Modules that are designed for use via from M import * should use the all mechanism to prevent exporting globals, or use the older convention of prefixing such globals with an underscore (which you might want to do to indicate these globals are “module non-public”).

and Constants:

Constants are usually defined on a module level and written in all capital letters with underscores separating words. Examples include MAX_OVERFLOW and TOTAL.

What do you think about making instances either _instances or INSTANCES (and same for registry).

[vidartf]

To fully close #3562 I think we should also add a note to the migration guide about this.

[jasongrout]

@maartenbreddels - I switched the internal imports to relative imports, and added a note to the migration docs like Vidar suggested in maartenbreddels#2 - can you take a look?

[bollwyvl]

This introduced a small issue, referring to register.get instead of _registry.get, which breaks creating new widgets in the client.

A prospective fix is up in #3653.