Tk event loop not working on Windows

Hi,

There seems to be a problem with the event loop magic for Tk on Windows.
Executing `%gui tk` in the kernel results in:
```
ERROR:tornado.application:Exception in callback functools.partial(.null_wrapper at 0x000000000
Traceback (most recent call last):
File "c:\program files\python37\lib\site-packages\tornado\ioloop.py", line 758, in _run_callback
ret = callback()
File "c:\program files\python37\lib\site-packages\tornado\stack_context.py", line 300, in null_wrapper
return fn(*args, **kwargs)
File "c:\program files\python37\lib\site-packages\ipykernel\kernelbase.py", line 295, in advance_eventloop
eventloop(self)
File "c:\program files\python37\lib\site-packages\ipykernel\eventloops.py", line 246, in loop_tk
app.tk.createfilehandler(stream.getsockopt(zmq.FD), READABLE, notifier)
AttributeError: '_tkinter.tkapp' object has no attribute 'createfilehandler'
```
Tested with
- Windows 7 x64
- Python 3.7.0 x64
- ipykernel 5.0.0

ipykernel 4.10 works fine. Also commenting out line 246 in eventloops.py seems to work.

Best,
Matthias