Python3: define a __next__() method for ModelBase

In Python 3, __next__() has replaced next().

Blueprint: make-python3-compatible
Change-Id: I96a7a4602be92cf78d4926bc4afd66bda9789395
Updated from global requirements

Change-Id: I8cf19fe3f96ade23d86c5d9306f4c9f582ef4630
Merge "Python3: define a __next__() method for ModelBase"
Merge "Updated from global requirements"