Fix issue #79

In Python 3.7, it's bad form to raise StopIteration at the end of a loop - and it yells at you for it by turning the exception into a Runtime Exception.

This simply removes throwing this, since it should be safe to just return.

Tested with pipenv on 2.7 and 3.7. We don't officially support 3.7 yet due to https://bugs.python.org/issue34226, but we should be close - support for everything except attachments.