title: Python multiprocessing, using pool multiple times in a loop gets stuck after first iteration
body: <p>I have the following situation, where I create a pool in a for loop as follows (I know it's not very elegant, but I have to do this for pickling reasons). Assume that the <code>pathos.multiprocessing</code> is equivalent to python's <code>multiprocessing</code> library (as it is up to some details, that are not relevant for this problem). 
  I have the following code I want to execute:</p>
  
  <pre><code>self.pool = pathos.multiprocessing.ProcessingPool(number_processes)
  
  
  for i in range(5):
  
  
      all_responses = self.pool.map(wrapper_singlerun, range(self.no_of_restarts))
  
      pool._clear()
  </code></pre>
  
  <p>Now my problem: The loop successfully runs the first iteration. However, at the second iteration, the algorithm suddenly stops (Does not finish the <code>pool.map</code> operation. I suspected that zombie processes are generated, or that the process was somehow <code>switched</code>. Below you will find everything I have tried so far.</p>
  
  <pre><code>for i in range(5):
  
      pool = pathos.multiprocessing.ProcessingPool(number_processes)
  
      all_responses = self.pool.map(wrapper_singlerun, range(self.no_of_restarts))
  
      pool._clear()
  
      gc.collect()
  
      for p in multiprocessing.active_children():
          p.terminate()
          gc.collect()
  
      print("We have so many active children: ", multiprocessing.active_children()) # Returns []
  </code></pre>
  
  <p>The above code works perfectly well on my mac. However, when I upload it on the cluster that has the following specs, I get the error that it gets stuck after the first iteration:</p>
  
  <pre><code>DISTRIB_ID=Ubuntu
  DISTRIB_RELEASE=18.04
  DISTRIB_CODENAME=bionic
  DISTRIB_DESCRIPTION="Ubuntu 18.04 LTS"
  </code></pre>
  
  <p>This is <a href="https://github.com/uqfoundation/pathos/blob/master/pathos/multiprocessing.py" rel="noreferrer">the link to the pathos</a>' multiprocessing library file is </p>