Respect Notebook nbformat version

[martinRenou]

🐛 Bug

It looks like papermill uses nbformat.v4 in multiple places of the code.

It should probably respect the nbformat that the Notebook being processed uses (looking at the nbformat and nbformat_minor versions from the Notebook)?

[martinRenou]

I have a more specific case happening locally:

Papermill is modifying my Notebook v4.4 ("id" is not a valid cell property for this version).
It adds a new cell to the Notebook with nbformat.v4.new_code_cell, which adds an "id" to this cell (a v4.5 cell). I suppose it cannot do better than this, because AFAICT nbformat has no way to do something like nbformat.v4.4.new_code_cell.
My Notebook ends up invalid for v4.4, so papermill should probably bump my Notebook version to v4.5.

[martinRenou]

Which means the wishlist for this issue is the following:

• Use nbformat.v*.new_code_cell respecting the Notebook's nbformat version
• Bump the Notebook's nbformat_minor version

[MSeal]

@martinRenou I agree and at this point 4.4 is quite old of a format and I don't see any reason to not just upgrade notebooks to 4.5 upon execution if they are older.

[martinRenou]

I can make a PR :)