title: `TypeError: 'NoneType' object is not iterable` in `subprocess.call`
body: <p>I am trying to apply <code>subprocess.call</code> instead of <code>os.system</code> following PEP 324</p>
  
  <p>In a task to open multiple urls</p>
  
  <pre><code>import subprocess
  open_chromes = [
  'https://en.wikipedia.org/wiki/Embodied_cognition',
  'https://docs.python.org/3.6/index.html',
  'https://docs.djangoproject.com/en/1.11/',]
  for chrome in open_chromes:
      cmd = ['open', '-na', 'Google Chrome']
      subprocess.call(cmd.append(chrome))
  </code></pre>
  
  <p>Error reported as</p>
  
  <pre><code>TypeError: 'NoneType' object is not iterable
  </code></pre>
  
  <p>Alternatively with <code>os.system</code> is definitely easy.</p>
  
  <pre><code>import os
  open_chromes = [
  'https://en.wikipedia.org/wiki/Embodied_cognition',
  'https://docs.python.org/3.6/index.html',
  'https://docs.djangoproject.com/en/1.11/',]
  
  for chrome in open_chromes:
      os.system('open -na "Google Chrome" {}'.format(chrome))
  </code></pre>
  
  <p>What's the problem with my code?</p>