run pre/post handler hooks around eventloop dispatch

ensures signal handlers are properly restored after exiting eventloop

ipykernel/kernelbase.py

@@ -257,7 +257,7 @@ def enter_eventloop(self):
  # which may be skipped by entering the eventloop
  stream.flush(zmq.POLLOUT)
  # restore default_int_handler
- signal(SIGINT, default_int_handler)
+ self.pre_handler_hook()
  while self.eventloop is not None:
  try:
  self.eventloop(self)
@@ -269,6 +269,7 @@ def enter_eventloop(self):
  # eventloop exited cleanly, this means we should stop (right?)
  self.eventloop = None
  break
+ self.post_handler_hook()
  self.log.info("exiting eventloop")
 
  def start(self):