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.

🤔 a BTree can not contain str and bytes as keys (the same way that on
python2 it can not contain unicode and str), basically we just need a
consistent type and something compatible with how we transform the data
from python2 databases.

TODO: use a slapos egg patch for py3 ... but do we still need this patch ?

"unicode" does not exist on python3 and on python2 it was not needed
anymore here.