Fix readthedocs build

[IsabelParedes]

This PR fixes the jupyterlite_sphinx error when building documentation

Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/jupyterlab-urdf/envs/latest/lib/python3.9/site-packages/jupyterlite_sphinx/jupyterlite_sphinx.py", line 121, in __init__
    super().__init__(rawsource, *children, iframe_src=iframe_src, **attributes)
TypeError: jupyterlite_sphinx.jupyterlite_sphinx._PromptedIframe.__init__() got multiple values for keyword argument 'iframe_src'
The full traceback has been saved in /tmp/sphinx-err-ipzrf2gu.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

[github-actions]

👈 Launch a Binder on branch ihuicatl/jupyterlab-urdf/docs