Installing the python extension should use the 'extension.open' command instead of opening the URL

[rchiodo]

See @jakebailey 's PR here:
microsoft/vscode-python#15321

[IanMatthewHuff]

Does this need to be tested on codespaces? I gave it a try locally and it worked popping up the same page as before. Did find one new issue that I filed separate: #4724. @rchiodo

[rchiodo]

Yeah it would be nice to test on codespaces. @joyceerhl can you try it out?

[rchiodo]

To validate:

1. Install just jupyter extension
2. Export from the interactive window.

[joyceerhl]

Webviews seem to all be busted in Codespaces right now, but if I install the Jupyter extension, create a blank native notebook and hit 'yes' on the prompt which recommends installing the Python extension, the install page correctly shows up in my Codespace.