feat(anywidget): Drop ipywidgets dependency

[manzt]

Important

This is an experiment to tame the Python complexity of anywidget. We want to avoid breaking API changes for most anywidget projects; but I anticipate this will result in some breaking changes for widgets using low-level (undocumented) APIs.

The idea is to have anywidgets continue to subclass anywidget.AnyWidget in the same exact way, but cut out ipywidgets.DOMWidget from the inheritance chain (favoring traitlets.HasTraits instead).

This PR also allows us to dogfood the anywidget.MimeBundleDescriptor work. One challenge with having widgets subclass from ipywidgets.DOMWidget is that all public (low-level) methods for creating widgets end up in public APIs to end users (i.e., poor encapsulation of comms).

Rather than polluting the top-level Widget with comms-related public methods, all comms usage is confined to the _repr_mimebundle_. This namespace is "private", and something that only anywidget authors would need to know about.

Before

❮ uv pip install -e .
Resolved 24 packages in 302ms
Installed 24 packages in 50ms
 + anywidget==0.9.11 (from file:///Users/manzt/github/manzt/anywidget)
 + asttokens==2.4.1
 + comm==0.2.2
 + decorator==5.1.1
 + exceptiongroup==1.2.1
 + executing==2.0.1
 + ipython==8.18.1
 + ipywidgets==8.1.2
 + jedi==0.19.1
 + jupyterlab-widgets==3.0.10
 + matplotlib-inline==0.1.7
 + parso==0.8.4
 + pexpect==4.9.0
 + prompt-toolkit==3.0.43
 + psygnal==0.11.1
 + ptyprocess==0.7.0
 + pure-eval==0.2.2
 + pygments==2.18.0
 + six==1.16.0
 + stack-data==0.6.3
 + traitlets==5.14.3
 + typing-extensions==4.11.0
 + wcwidth==0.2.13
 + widgetsnbextension==4.0.10

After

❮ uv pip install -e .
   Built file:///Users/manzt/github/manzt/anywidget
Built 1 editable in 1.67s
Resolved 7 packages in 159ms
Installed 7 packages in 12ms
 + anywidget==0.9.11 (from file:///Users/manzt/github/manzt/anywidget)
 + comm==0.2.2
 + jupyterlab-widgets==3.0.10 # ensures widgets work in JupyterLab
 + psygnal==0.11.1
 + traitlets==5.14.3
 + typing-extensions==4.11.0
 + widgetsnbextension==4.0.10 # ensures widgets works in Classic Notebooks

TODO:

• Custom serializers
• Unit tests

FWIW, I think ideally widget authors would have more choice over the class for their widget (i.e., msgspec, pydantic, dataclass), but this is definitely a step in the right direction. If anything, it allows us to nail down APIs on the mimebundle descriptor that are "essential".

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