Update for JupyterLab 3.0

[jtpio]

At the moment this extension only supports JupyterLab 2.0:

https://github.com/jtpio/jupyterlab-python-bytecode/blob/23c3d4e228829743b65685d8796cf1bc95fd88ba/package.json#L36

It would be great if it could be updated for JupyterLab 3.0 and adopt the prebuilt extension system:

Linking to a couple of useful resources:

• Using the upgrade script: https://jupyterlab.readthedocs.io/en/stable/extension/extension_migration.html#upgrading-library-versions-using-the-upgrade-script
• Extension tutorial using the Python packaging for distributing the extension: https://jupyterlab.readthedocs.io/en/stable/extension/extension_tutorial.html
• Docs on the prebuilt extensions: https://jupyterlab.readthedocs.io/en/stable/extension/extension_dev.html#prebuilt-extensions