Jupyter improvements/modernisation

[Atreus-Technologies]

I'm moving a lot of my Python work over to Rust and was checking whether plotly was going to work with the Rust kernel for Jupyter evcxr.

There's a bit of documentation as to how this works in this repo but it wasn't immediately clear that plotly.rs was going to work with modern Jupyter setups based on

1. the references to jupyter labextension install, which is no longer required.
2. extern crate mentions, not required anymore.
3. Arcane and unecessary EVCXR_BEGIN_CONTENT content here
4. requirements for ipywidgets, not required AFAICT here

When trying to determine whether this library was going to work with modern jupyter versions, the tests didn't quite convince me because they are just smoke tests that ensure the methods don't panic and might not check conformance with any changes expected in the Jupyter API (there aren't any in this case, but it is good to check and might be nice to have a reference to the Jupyter version that the tests are targeting).

Happy to raise a PR if you folks would like me to, nothing dramatic here just a few things that would give users of modern Jupyter lab versions some immediate confidence that the library will work.

[andrei-ng]

Hi @Atreus-Technologies,

Please do raise a PR if you have the time. Would appreciate it!

Thanks!