Test if is_nonstr_iter works for numpy arrays

[seisman]

Description of proposed changes

Add a doctest checking if function is_nonstr_iter works for numpy arrays.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[weiji14]

By the way, maybe add the skip-changelog label since this is a pretty small change.