Raise on Python 3.
`io.` or `codec.open` maybe ?
```
  File "/home/travis/virtualenv/python3.3/lib/python3.3/site-packages/ipython-2.0.0_dev-py3.3.egg/IPython/core/magic.py", line 192, in <lambda>
    call = lambda f, *a, **k: f(*a, **k)
  File "/home/travis/virtualenv/python3.3/lib/python3.3/site-packages/ipython-2.0.0_dev-py3.3.egg/IPython/extensions/cachemagic.py", line 74, in cache
    cache = pickle.load(f)
  File "/home/travis/virtualenv/python3.3/lib/python3.3/codecs.py", line 300, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
nose.proxy.UnicodeDecodeError: UnicodeDecodeError: utf-8
```