csv class writer : a bytes-like object is required, not 'str'

For a personal project I'm trying to upgrade the paterns package to Python 3. Actually I'm running the test:db.py, but I'm stuck with the following error in the __init__.py file, on a csv class:

This is the code snippet od the save() function: there, we dfine s as a BytesIO() stream, so the function is asked to stream bytes to a self csv file. The error comes from the line:

w.writerows([[csv_header_encode(name, type) for name, type in self.fields]])

TypeError: a bytes-like object is required, not 'str' ( below, also the code for this function)

it's supposed that csv_header_encode delivers bytes, and I checked this and it does, but somehow, in its conversion to list it changes to 'str'. Ad if I change the s encoding to StringsIO then the complaining comes from

 f.write(BOM_UTF8)

Any help will be appreciated.

def save(self, path, separator=",", encoder=lambda v: v, headers=False, password=None, **kwargs):
    """ Exports the table to a unicode text file at the given path.
        Rows in the file are separated with a newline.
        Columns in a row are separated with the given separator (by default, comma).
        For data types other than string, int, float, bool or None, a custom string encoder can be given.
    """
    # Optional parameters include all arguments for csv.writer(), see:
    # http://ift.tt/2phH4ii
    kwargs.setdefault("delimiter", separator)
    kwargs.setdefault("quoting", csvlib.QUOTE_ALL)
    # csv.writer will handle str, int, float and bool:
    s = BytesIO()
    w = csvlib.writer(s,  **kwargs)
    if headers and self.fields is not None:
        w.writerows([[csv_header_encode(name, type) for name, type in self.fields]])
    w.writerows([[encode_utf8(encoder(v)) for v in row] for row in self])
    s = s.getvalue()
    s = s.strip()
    s = re.sub("([^\"]|^)\"None\"", "\\1None", s)
    s = (s if not password else encrypt_string(s, password)).encode('latin-1')
    f = open(path, "wt")
    f.write(BOM_UTF8)
    f.write(s)
    f.close()

def csv_header_encode(field, type=STRING):
    # csv_header_encode("age", INTEGER) => "age (INTEGER)".
    t = re.sub(r"^varchar\(.*?\)", "string", (type or ""))
    t = t and " (%s)" % t or ""
    return "%s%s" % (encode_utf8(field or ""), t.upper())