Try to fix Qt event loop, take III

This involves neither threads nor calling in to the old inputhook
machinery.

IPython/terminal/pt_inputhooks/qt.py

@@ -1,3 +1,4 b''
+import sys
 from IPython.external.qt_for_kernel import QtCore, QtGui
 
 def inputhook(context):
@@ -5,6 +6,19 b' def inputhook(context):'
     if not app:
         return
     event_loop = QtCore.QEventLoop(app)
+
+    if sys.platform == 'win32':
+        # The QSocketNotifier method doesn't appear to work on Windows.
+        # Use polling instead.
+        timer = QtCore.QTimer()
+        timer.timeout.connect(event_loop.quit)
+        while not context.input_is_ready():
+            timer.start(50)  # 50 ms
+            event_loop.exec_()
+            timer.stop()
+    else:
+        # On POSIX platforms, we can use a file descriptor to quit the event
+        # loop when there is input ready to read.
     notifier = QtCore.QSocketNotifier(context.fileno(), QtCore.QSocketNotifier.Read)
     notifier.setEnabled(True)
     notifier.activated.connect(event_loop.exit)