Version 0.5.0 not working on classic notebook

[benbovy]

To reproduce (using classic notebook v6.5.4):

$ python -m pip install notebook ipytone==0.5.0
$ jupyter nbextension list
Known nbextensions:
  config dir: /Users/me/miniconda3/envs/temp/etc/jupyter/nbconfig
    notebook section
      ipytone/extension  enabled
      - Validating: problems found:
        - require?  X ipytone/extension
      jupyter-js-widgets/extension  enabled
      - Validating: OK

Jupyterlab extension is working fine.

A similar issue is reported here: jupyter-widgets/widget-ts-cookiecutter#131 (EDIT: not related, the issue here is the nbextension js files put in nbextension/static instead of nbextension/, not sure why).