title: Subclass `pathlib.Path` fails
body: <p>I would like to enhance the class <code>pathlib.Path</code> but the simple example above dose not work.</p>
  
  <pre><code>from pathlib import Path
  
  class PPath(Path):
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
  
  test = PPath("dir", "test.txt")
  </code></pre>
  
  <p>Here is the error message I have.</p>
  
  <pre><code>Traceback (most recent call last):
    File "/Users/projetmbc/test.py", line 14, in &lt;module&gt;
      test = PPath("dir", "test.txt")
    File "/anaconda/lib/python3.4/pathlib.py", line 907, in __new__
      self = cls._from_parts(args, init=False)
    File "/anaconda/lib/python3.4/pathlib.py", line 589, in _from_parts
      drv, root, parts = self._parse_args(args)
    File "/anaconda/lib/python3.4/pathlib.py", line 582, in _parse_args
      return cls._flavour.parse_parts(parts)
  AttributeError: type object 'PPath' has no attribute '_flavour'
  </code></pre>
  
  <p>What I am doing wrong ?</p>