Support new JupyterLab 3.0 python based packaging

[yuvipanda]

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Hello! Very helpful extension, thank you for building and publishing it!

If I install the gmaps python package and open JupyterLab, I get this:

JupyterLab 3.0 lets you distribute extensions without needing npm or node. I'd love for gmaps to support that, so we don't have to run a build step after installing the python package.

[gbrault]

@yuvipanda is right: the extension is awesome but need some upgrade to run on lav v3.0. Would be wonderful to update it! Thanks.