Draft: erp5_big_file: allow to continue store as string in python3

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 ?