i realize because of https://lab.nexedi.com/nexedi/erp5/-/commit/1b555dbf6827d420ffd594a2308c4b273035c01d?page=3 of big file
in python3, we have to pass data as bytes instead of string
which makes old object has mixed data type: bytes, string when upgrade from python2 to python3
maybe we should to allow to store as string as before
@jerome what do you think ?