title: Don't understand this SyntaxError about the metaclass
body: <p>Codes below are atomic.py from the pyrlp package, which is a library of pyethereum.</p>
  
  <pre><code>import abc
  
  
  class Atomic(metaclass=abc.ABCMeta):
      """ABC for objects that can be RLP encoded as is."""
      pass
  
  
  Atomic.register(bytes)
  Atomic.register(bytearray)
  </code></pre>
  
  <p>And here is the error information from the console:</p>
  
  <pre class="lang-none prettyprint-override"><code>from rlp.atomic import Atomic
    File "/Users/jerryin/Desktop/pyeth/pyrlp/rlp/atomic.py", line 4
      class Atomic(metaclass=abc.ABCMeta):
                            ^
  SyntaxError: invalid syntax
  </code></pre>
  
  <p>I've not modified the project since it was imported. Could anyone please tell me what's wrong with this?</p>
  
  <p>Plus, the interpreter I used for this project is Python 2.7.</p>