Get versioned Notebook framework trove classifiers on PyPI

[bollwyvl]

A while back, JupyterLab was able to get some custom trove classifiers on PyPI.

pypa/trove-classifiers#72

It would be useful for extensions that support Notebook 7 to be able to advertise this directly, and this will likely be one of the sources that any kind of magic extension installing tools (shudder) would use.

Here are the proposed ones:

Framework :: Jupyter :: Notebook
Framework :: Jupyter :: Notebook :: 6
Framework :: Jupyter :: Notebook :: 7

I would think that it's probably unnecessary to duplicate the Themes, Prebuilt and Mime Renderers classifiers, as under the hood, these will still be mostly defined with labextension semantics, and these specific kinds will almost certainly work with JupyterLab though this may complicate "casual" browsing on PyPI.