Compatibility of tests with ipywidgets 8 beta 1

Hello.



I'm testing also ipywidgets 8 beta 1 and it seems that there is only one problem. The new version of ipywidgets produces a different output in the notebook than what we expect in `test_run_all_notebooks[JupyterWidgets.ipynb-opts7]`:

```

>           assert normalized_expected_outputs == normalized_actual_outputs

E           AssertionError: assert [{'data': {'application/vnd.jupyter.widget-view+json': {'model_id': '<MODEL_ID>',\n                                                        'version_major': 2,\n                                                        'version_minor': 0},\n           'text/plain': 'LabelvalueHelloWorld'},\n  'output_type': 'display_data'}] == [{'data': {'application/vnd.jupyter.widget-view+json': {'model_id': '<MODEL_ID>',\n                                                        'version_major': 2,\n                                                        'version_minor': 0},\n           'text/plain': 'LabelvalueHelloWorld'},\n  'execution_count': 1,\n  'output_type': 'execute_result'}]

E             At index 0 diff: {'data': {'application/vnd.jupyter.widget-view+json': {'model_id': '<MODEL_ID>', 'version_major': 2, 'version_minor': 0}, 'text/plain': 'LabelvalueHelloWorld'}, 'output_type': 'display_data'} != {'output_type': 'execute_result', 'data': {'text/plain': 'LabelvalueHelloWorld', 'application/vnd.jupyter.widget-view+json': {'version_major': 2, 'version_minor': 0, 'model_id': '<MODEL_ID>'}}, 'execution_count': 1}

E             Full diff:

E               [

E                {'data': {'application/vnd.jupyter.widget-view+json': {'model_id': '<MODEL_ID>',

E                                                                       'version_major': 2,

E                                                                       'version_minor': 0},

E                          'text/plain': 'LabelvalueHelloWorld'},

E             +   'output_type': 'display_data'},

E             -   'execution_count': 1,

E             -   'output_type': 'execute_result'},

E               ]



nbclient/tests/test_client.py:231: AssertionError

```

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