Return extracted traceback frame 4-tuples that aren't unittest ones.

    This used to be _exc_info_to_string().

def extract_relevant_tb(tb, exctype, is_test_failure):
    """Return extracted traceback frame 4-tuples that aren't unittest ones.

    This used to be _exc_info_to_string().

    """
    # Skip test runner traceback levels:
    while tb and _is_unittest_frame(tb):
        tb = tb.tb_next
    if is_test_failure:
        # Skip assert*() traceback levels:
        length = _count_relevant_tb_levels(tb)
        return extract_tb(tb, length)
    return extract_tb(tb)