title: Python3: TypeError: unhashable type: 'list' when using Counter
body: <p>I am using Python 3 and I have this code:</p>
  
  <pre><code>from collections import Counter
  c = Counter([r[1] for r in results.items()])
  </code></pre>
  
  <p>But when I run it I get this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;pyshell#100&gt;", line 1, in &lt;module&gt;
      c = Counter([r[1] for r in results.items()])
    File "C:\Python33\lib\collections\__init__.py", line 467, in __init__
      self.update(iterable, **kwds)
    File "C:\Python33\lib\collections\__init__.py", line 547, in update
      _count_elements(self, iterable)
  TypeError: unhashable type: 'list'
  </code></pre>
  
  <p>Why am I getting this error? The code was originally written for Python 2, but I am using it in Python 3. Is there anything that changed between Python 2 and 3?</p>