- Matches changes in Python 3.x dictionary methods.
- Iterating now raises a run-time error if the property-group changes
size during iteration.
- IDPropertyGroup.iteritems() has been removed.
- IDPropertyGroup View & Iterator types have been added.
- Some set functionality from dict_keys/values/items aren't yet
supported (isdisjoint method and boolean set style operations).
Proposed as part of T85675.