Doc build failure - unicode error in generate_example_rst

On Python 2, `io.open()` is called without an encoding argument. The default encoding it uses depends on the locale, so on Launchpad buildbots using the C locale, the default encoding is ASCII, and it chokes on any non-ascii character.

(This platform-dependent default is really annoying for anything cross platform, but it's part of the tension between Python as a serious application language and Python as a quick scripting language)

```
Exception occurred:
  File "/usr/lib/python2.7/dist-packages/sphinx/cmdline.py", line 188, in main
    warningiserror, tags)
  File "/usr/lib/python2.7/dist-packages/sphinx/application.py", line 134, in __init__
    self._init_builder(buildername)
  File "/usr/lib/python2.7/dist-packages/sphinx/application.py", line 195, in _init_builder
    self.emit('builder-inited')
  File "/usr/lib/python2.7/dist-packages/sphinx/application.py", line 314, in emit
    results.append(callback(self, *args))
  File "/build/buildd/matplotlib-1.2.0~1+6540+23~raring1/doc/sphinxext/gen_rst.py", line 44, in generate_example_rst
    contents = io.open(fullpath).read()
  File "/usr/lib/python2.7/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
```

Buildlog: https://launchpadlibrarian.net/137176232/buildlog_ubuntu-raring-i386.matplotlib_1.2.0~1%2B6540%2B23~raring1_FAILEDTOBUILD.txt.gz