'TarFile' object has no attribute 'peel'

When I try the sample code:

``` python
>> import tarfile, pygit2
>> with tarfile.open('foo.tar', 'w') as archive:
        repo = pygit2.Repsitory('.')
        repo.write_archive(archive, repo.head.target)
```

an exception raised:
**'TarFile' object has no attribute 'peel'**

indeed, tarfile didn't have the `peel` attr.
