Compatibility with ipywidgets 8.x

[Carifio24]

This PR aims to make the pywwt widget compatible with ipywidgets 8.x, which currently seems to not be the case. I was trying to use the Jupyter widget in a glue-wwt context together with some of the new widgets from ipywidgets 8 but kept getting the following error:

Model class 'WWTModel' from module 'pywwt' is loaded but can not be instantiated
TypeError: r._deserialize_state is not a function
    at f._make_model (http://localhost:8889/lab/extensions/@jupyter-widgets/jupyterlab-manager/static/150.b0e841b75317744a7595.js?v=b0e841b75317744a7595:1:7933)

After some investigation, it seems that we need to update our widget JS to use ES6 class notation (see here), which is the content of this PR. With these changes, the widget works for me locally with both ipywidgets 7.x and 8.x.

[codecov]

Codecov Report

Merging #368 (c90b86e) into master (8361ac1) will not change coverage.
The diff coverage is n/a.

❗ Current head c90b86e differs from pull request most recent head b20b820. Consider uploading reports for the commit b20b820 to get more accurate results

@@           Coverage Diff           @@
##           master     #368   +/-   ##
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  Coverage   56.51%   56.51%           
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  Files          25       25           
  Lines        2870     2870           
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  Hits         1622     1622           
  Misses       1248     1248           

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[pkgw]

Thanks for tracking this down!