Applying minor version upgrades to notebook

[nicholaswold]

I ran into this problem while trying to use #189.

Right now, upgrade doesn't actually apply minor version changes to the given notebook (ref: https://github.com/jupyter/nbformat/blob/master/nbformat/v4/convert.py#L67).

I was unable to find a method that could apply minor version updates on existing notebooks without downgrading first (ping @willingc who was on call trying to help me find a method.) writes and reads appear to concern themselves only with major versions, and upgrade_cell is specifically stated to only work with v3 cells and will break v4 cells if provided.

If there is no method to do minor version migrations, then my proposal would be that this block of code should look something like this:

if from_minor == nbformat_minor:
    return

# other versions migration code e.g.
# if from_minor < 3:
# if from_minor < 4:

if from_minor < 5:
    for cell in nb.cells:
        cell.id = random_cell_id()

nb.metadata.orig_nbformat_minor = from_minor
nb.nbformat_minor = nbformat_minor

By doing this, upgrade would be able to apply minor-version patches step-by-step for notebooks within the same major-version.

[MSeal]

I had though this had been addressed, causing me to release 5.1.0 -- we should add this fix and do a 5.1.1 release to improve the upgrade to care about minor 4.x versions.