start_new_session was added to python3 subprocess in v3.2 and
is meant to take the place of the common use of preexec_fn to
call os.setsid() in the child - as done here.
Update to use the new equivalent.
(From OE-Core rev: 0ebb56ff192e9c6db2af83c37db3307a27fe822a)
Signed-off-by: André Draszik <git@andred.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>