title: Simple multiprocessing Pool hangs in Jupyter notebook
body: <p>I'm trying to run some multiprocessing in a Jupyter notebook, using python version 3.7.0.
  However, even a really simple example seems to hang indefinitely.
  After reading <a href="https://stackoverflow.com/a/56780184/3752268">this answer</a> I tried explicitly calling .close and .join on the pool, but it still hangs. Example code is below, can anyone tell me what's wrong?</p>
  <pre><code>import multiprocessing as mp
  
  def fun(x):
      return 2*x
  
  with mp.Pool() as pool:
      args = list(range(10))
      res = pool.map(fun, args)
      pool.close()
      pool.join()
  </code></pre>