Having a form which allows sending emails with arbitrary senders,
recipients, and content, sent using the site's mail setup, is a recipe for
sending spam and destroying one's mail system's reputation.

See merge request nexedi/erp5!1872

Should be kept enough abstract so it can be used to make any RESTAPI request

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.