ensure ``arg`` is a list of strings

def _validate_str_list(arg):
    '''
    ensure ``arg`` is a list of strings
    '''
    if isinstance(arg, six.binary_type):
        ret = [salt.utils.stringutils.to_unicode(arg)]
    elif isinstance(arg, six.string_types):
        ret = [arg]
    elif isinstance(arg, Iterable) and not isinstance(arg, Mapping):
        ret = []
        for item in arg:
            if isinstance(item, six.string_types):
                ret.append(item)
            else:
                ret.append(six.text_type(item))
    else:
        ret = [six.text_type(arg)]
    return ret