aq_base(obj) instead of obj.aq_base

[jensens]

For some reason at
https://github.com/plone/plone.app.upgrade/blob/master/plone/app/upgrade/v50/final.py#L73
the obj may not have an acquisition-wrapper (due to prior problems obj is a list in our case, but this must not stop the migration process here). So attribute access fails and migration breaks.

Using aq_base(obj) instead of obj.aq_base is considered best practice and has to be used here as well.