Make a separator for a prose-like list with `,` between items except
    for `, and` after the second to last item.

def _oxford_comma_separator(i, length):
    """Make a separator for a prose-like list with `,` between items except
    for `, and` after the second to last item.
    """
    if length == 1:
        return None
    elif length < 3 and i == 0:
        return ' and '
    elif i < length - 2:
        return ', '
    elif i == length - 2:
        return ', and '
    else:
        return None