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, for consistency with other BTrees, such as the
ones used for OFS.
group_id is also used in a SQL column which is BINARY, this is
problematic on py3 because the selected values will be returned as bytes,
but we expect str here. Because we don't want to run a data migration,
we adjust the select methods to convert to str while selecting.

Since years there was a warning that id_group must be a string, now we
make it a bit stricter, we also enforce that the id_group is valid UTF-8.

A few more tests and assertions were also added.

Reviewed-on: nexedi/erp5!1980