Update PyPi Homepage links

[dmadisetti]

https://pypi.org/project/observable-jupyter/

currently points to the dead https://github.com/observablehq/observable_jupyter/

[dmadisetti]

Also if you could cut a new release, that would be amazing 🎉

Thanks for maintaining the project

[thomasballinger]

I'll take a look after work, ping me again if this doesn't happen by tomorrow evening!