Run the sample code to report an error

[alien10086]

python 3.6.5 pyppeteer 0.0.23 macbook pro high sierra 10.13.6

from pyppeteer import launch
async def main():
    browser = await launch()
    page = await browser.newPage()
    await page.goto('http://example.com')
    await page.screenshot({'path': 'example.png'})
    await browser.close()

asyncio.get_event_loop().run_until_complete(main())```

```Task exception was never retrieved
future: <Task finished coro=<Connection._recv_loop() done, defined at /usr/local/lib/python3.6/site-packages/pyppeteer/connection.py:54> exception=InvalidState('Cannot write to a WebSocket in the CLOSING state',)>
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/pyppeteer/connection.py", line 66, in _recv_loop
    await asyncio.sleep(0)
  File "/usr/local/lib/python3.6/site-packages/websockets/client.py", line 390, in __aexit__
    yield from self.ws_client.close()
  File "/usr/local/lib/python3.6/site-packages/websockets/protocol.py", line 370, in close
    self.timeout, loop=self.loop)
  File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/asyncio/tasks.py", line 358, in wait_for
    return fut.result()
  File "/usr/local/lib/python3.6/site-packages/websockets/protocol.py", line 642, in write_frame
    "in the {} state".format(self.state.name))
websockets.exceptions.InvalidState: Cannot write to a WebSocket in the CLOSING state```

[miyakogi]

I have tested with python 3.6.5 on OS X high sierra 10.13.6, but not reproduced.