Change generator termination from StopIteration to return.

Change generator termination from StopIteration to return.

From [PEP 479](https://www.python.org/dev/peps/pep-0479/) the correct way to
terminate a generator is to return (which implicitly raises StopIteration)
rather than raise StopIteration.

Without this change using sqlalchemy in python 3.5 or greater results in
these warnings
    PendingDeprecationWarning: generator '__iter__' raised StopIteration
which this commit should remove.