Backport PR #4639: Minor import fix to get qtconsole with --pylab=qt working

Running the qtconsole with --pylab=qt on non-OSX-10.9 OSes fails with
the error `TypeError: 'NoOpContext' object is not callable`.

IPython/kernel/zmq/eventloops.py

@@ -40,7 +40,7 b' def _notify_stream_qt(kernel, stream):'
     if _on_os_x_10_9() and kernel._darwin_app_nap:
         from IPython.external.appnope import nope_scope as context
     else:
-        from IPython.core.interactiveshell import no_op_context as context
+        from IPython.core.interactiveshell import NoOpContext as context
 
     def process_stream_events():
         while stream.getsockopt(zmq.EVENTS) & zmq.POLLIN: