title: Zipping a product of iterators and a count iterator
body: <p>I am trying to concatenate a count to the product of two iterators as follows:</p>
  
  <pre><code>import itertools
  it1 = itertools.product(['A', 'B'], [1, 2, 3])
  it2 = itertools.count(1)
  list(zip(it1, it2))
  </code></pre>
  
  <p>This generates the output</p>
  
  <pre><code>[(('A', 1), 1),
   (('A', 2), 2),
   (('A', 3), 3),
   (('B', 1), 4),
   (('B', 2), 5),
   (('B', 3), 6)]
  </code></pre>
  
  <p>However, what I would like to have is</p>
  
  <pre><code>[('A', 1, 1),
   ('A', 2, 2),
   ('A', 3, 3),
   ('B', 1, 4),
   ('B', 2, 5),
   ('B', 3, 6)]
  </code></pre>