Issue with typing.AscyncGenerator for python 3.6.0

Hi there,



I upgraded to `nbconvert 6.0.7` from from `5.6.1` recently, so as part of that I installed `nbclient 0.5.1`. 



```

python 3.6.0

typing 3.7.4.3

notebook 6.1.4

```



I'm running python 3.6.0 on this machine. After the upgraded I started getting these errors when I tried to start a notebook (500, internal error page):





```

#....

File "h:\anaconda3\lib\site-packages\nbclient\client.py", line447, in NotebookClient

  async def async_setup_kernel(self, **kwargs) -> t.AsyncGenerator:

AttributeError: module 'typing' has no attribute 'AsyncGenerator'

```



If I revert back to `nbconvert 5.6.1` life is back to normal again, notebooks can be opened. 



On another machine with python 3.7, I do not see this issue. This might be specifically related to `python 3.6.0`. 



Unfortunately I couldn't upgrade the machine to 3.7 easily... Any idea how / why this was happening to 3.6.0?



Thanks.

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