oeqa: modernise subprocess: preexec_fn=os.setsid -> start_new_session=True

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: c3bd502f31a621394a2f6ea3ae90443ff4ac98a4)

Signed-off-by: André Draszik <git@andred.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/utils/sshcontrol.py

@@ -23,7 +23,7 @@ class SSHProcess(object):
             "stdin": None,
             "shell": False,
             "bufsize": -1,
-            "preexec_fn": os.setsid,
+            "start_new_session": True,
         }
         self.options = dict(self.defaultopts)
         self.options.update(options)