[JupyROOT][8055] Make poller thread a daemon

As explained in 8055, it can otherwise keep a process spawned
from the kernel process from finishing.

Author: etejedor

bindings/jupyroot/python/JupyROOT/helpers/handlers.py

@@ -77,6 +77,7 @@ def GetStreamsDicts(self):
 class Poller(Thread):
     def __init__(self):
         Thread.__init__(self, group=None, target=None, name="JupyROOT Poller Thread")
+        self.daemon = True
         self.poll = True
         self.is_running = False
         self.queue = queue.Queue()