title: Python concurrent.futures: ProcessPoolExecutor fail to work
body: <p>I'm trying to use the ProcessPoolExecutor method but it fails.
  Here is an example(calculation of the big divider of two numbers) of a failed use.
  I don't understand what the mistake is</p>
  
  <pre><code>def gcd(pair):
      a, b = pair
      low = min(a, b)
      for i in range(low, 0, -1):
          if a % i == 0 and b % i == 0:
              return i
  
  numbers = [(1963309, 2265973), (2030677, 3814172),
             (1551645, 2229620), (2039045, 2020802)]
  start = time()
  pool = ProcessPoolExecutor(max_workers=2)
  results = list(pool.map(gcd, numbers))
  end = time()
  print('Took %.3f seconds' % (end - start))
  </code></pre>
  
  <p>BrokenProcessPool: A process in the process pool was terminated abruptly while the future was running or pending.</p>