`RuntimeError: generator raised StopIteration` in `stepvector.py`

[krooijers]

In an effort to reproduce #1 I encountered a RuntimeError due to the StopIteration being raised in stepvector.py

Some background reading:

• https://www.juandebravo.com/2018/11/09/python-37-stop-iteration/
• https://docs.python.org/3/whatsnew/3.7.html#changes-in-python-behavior
• https://peps.python.org/pep-0479/

Workaround was to either:

• return instead of raise StopIteration (as proposed in documentation)
• revert to python version < 3.7