Support Jupytext

[jitseniesen]

Integrate jupytext with the spyder-notebook plugin so a file in the Spyder editor could be opened in the notebook or a notebook could be opened in the editor. It looks like jupytext does most of the heavy lifting and it's mostly a user interface thing.

Originally posted by @bcolsen in spyder-ide/spyder#7933 (comment)

[aviolov]

I second this request. Note that a notebook can already be opened in the editor as a converted .py scripts (right click on ,ipynb and select 'Convert to Python script')

Now we just need a similar context-option for a .py script to 'Convert to Jupyter Notebook' ! Would be nice.