Get versioned Notebook framework trove classifiers on PyPI

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

https://github.com/pypa/trove-classifiers/pull/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](https://pypi.org/search/?q=&o=&c=Framework+%3A%3A+Jupyter+%3A%3A+JupyterLab+%3A%3A+4).

Start by reviewing pypa/trove-classifiers#72 and the three proposed Notebook classifiers in this issue. Then identify how the Notebook package metadata is published to PyPI and confirm the classifiers can be added there. Done means the versioned Notebook classifiers are accepted and available for extensions to use.

Written by the indexing model from the issue text.