With fromlist='erp5.component.extention' (that should be ['erp5.component.extention']), it works in Python 2.
But in Python 3, we have an exception like:
  ModuleNotFoundError: No module named 'erp5.component.extension.erp5_version.InventoryBrain.'
With just fromlist=[''], like we do in XMLSyncUtils, it works on both Python 2 and Python 3.

Jérome: slightly adjusted manually to just use range where it does not
make a significant difference (for example in test)

also str representation of unicode character is different.

Co-authored-by: Carlos Ramos Carreño <carlos.ramos@nexedi.com>
Co-authored-by: Emmeline Vouriot <emmeline.vouriot@nexedi.com>
Co-authored-by: Kazuhiko SHIOZAKI <kazuhiko@nexedi.com>

some conversion (TextDocument) depend on the content type, so it's
necessary to first update the content type before trying to convert to
base format

group_id is used as key of OOBtree and as documented, it's not
possible to mix keys that can not be compared, so we can not have a mix
of string and bytes. Since years