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.

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.

This improves readability when activity_kw, args or kw are big.