test notebooks are free from errors or warnings

[slayoo]

something like this:

def test_no_errors_or_warnings_in_output(notebook):
    with open(notebook) as fp:
        nb = nbformat.read(fp, nbformat.NO_CONVERT)
        for cell in nb.cells:
            if cell.cell_type == "code":
                for output in cell.outputs:
                    if "name" in output and output["name"] == "stderr":
                        raise AssertionError(output["text"])

[github-actions[bot]]

Stale issue message

[slayoo]

addressed in #19