text: Title: Add the JupyterLab trove classifiers
  Body: <!--
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  ### Problem
  
  The new trove classifiers for JupyterLab prebuilt extensions have just been added to PyPI. See the following issues and PRs for more context:
  
  - https://github.com/pypa/trove-classifiers/pull/72
  - https://github.com/jupyterlab/extension-cookiecutter-ts/pull/174
  
  This will help discover prebuilt JupyterLab extensions on PyPI. For example with this search request:
  
  https://pypi.org/search/?c=Framework+%3A%3A+Jupyter+%3A%3A+JupyterLab+%3A%3A+Extensions+%3A%3A+Prebuilt
  
  <!-- Provide a clear and concise description of what problem this feature will solve. For example:
  
  * I'm always frustrated when [...] because [...]
  * I would like it if [...] happened when I [...] because [...]
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  ### Proposed Solution
  
  We should add them to the `voila` package too here:
  
  https://github.com/voila-dashboards/voila/blob/56fe4cf4bfe114168819f065e22afcb760f51ac6/setup.py#L111-L120
  
  ```
  Framework :: Jupyter :: JupyterLab
  Framework :: Jupyter :: JupyterLab :: 3
  Framework :: Jupyter :: JupyterLab :: Extensions
  Framework :: Jupyter :: JupyterLab :: Extensions :: Prebuilt
  ```
  
  ### Additional context
  
  