WIP: Add support for CMF 2.3
This is part of the work to update ERP5 and its dependencies to support Python 3.
We incrementally update CMF to versions:
- 2.3 (this MR), the last version compatible with Zope 2.13.x
- 2.4+ (in the future), only compatible with Zope 4
We also drop Products.CMFDefault and Products.CMFUid, at the same time. These products are not compatible with Python 3.