prompt: We are currently solving the following issue within our repository. Here is the issue text:
  --- BEGIN ISSUE ---
  DictDataset throws an internal error only in Python 3.
  The following code throws an internal error only in Python 3 environment with the latest version of Chainer. (3.0.0b1, 8bcac6f)
  ```py
  from chainer.datasets import DictDataset
  
  def main():
      a = range(10)
      b = range(10, 20)
      dataset = DictDataset(x=a, y=b)
      print(dataset[0:5])
  
  if __name__ == '__main__':
      main()
  ```
  
  In Python 3.6.1,
  ```sh
  $ python --version
   Python 3.6.1
  $ python poc.py
   Traceback (most recent call last):
    File "poc.py", line 12, in <module>
      main()
    File "poc.py", line 8, in main
      print(dataset[0:5])
    File "/home/igarashi/projects/chainer/chainer/datasets/dict_dataset.py", line 34, in __getitem__
      length = len(six.itervalues(batches).next())
  AttributeError: 'dict_valueiterator' object has no attribute 'next'
  ```
  
  In Python 2.7.13, 
  ```sh
  $ python --version
   Python 2.7.13
  $ python poc.py
   [{'y': 10, 'x': 0}, {'y': 11, 'x': 1}, {'y': 12, 'x': 2}, {'y': 13, 'x': 3}, {'y': 14, 'x': 4}]
  ```
  
  It is because an instance of `six.Iterator` doesn't have `next()` method in the Python 3 environment.
  [Reference](http://pythonhosted.org/six/#six.Iterator)
  
  
  --- END ISSUE ---
  
  Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
  --- BEGIN FILES ---
  Path: `chainer/datasets/dict_dataset.py`
  Content:
  ```
  1 import six
  2 
  3 
  4 class DictDataset(object):
  5 
  6     """Dataset of a dictionary of datasets.
  7 
  8     It combines multiple datasets into one dataset. Each example is represented
  9     by a dictionary mapping a key to an example of the corresponding dataset.
  10 
  11     Args:
  12         datasets: Underlying datasets. The keys are used as the keys of each
  13             example. All datasets must have the same length.
  14 
  15     """
  16 
  17     def __init__(self, **datasets):
  18         if not datasets:
  19             raise ValueError('no datasets are given')
  20         length = None
  21         for key, dataset in six.iteritems(datasets):
  22             if length is None:
  23                 length = len(dataset)
  24             elif length != len(dataset):
  25                 raise ValueError(
  26                     'dataset length conflicts at "{}"'.format(key))
  27         self._datasets = datasets
  28         self._length = length
  29 
  30     def __getitem__(self, index):
  31         batches = {key: dataset[index]
  32                    for key, dataset in six.iteritems(self._datasets)}
  33         if isinstance(index, slice):
  34             length = len(six.itervalues(batches).next())
  35             return [{key: batch[i] for key, batch in six.iteritems(batches)}
  36                     for i in six.moves.range(length)]
  37         else:
  38             return batches
  39 
  40     def __len__(self):
  41         return self._length
  42
  ```
  --- END FILES ---
  
  Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.
  
  Here is an example:
  
  ```diff
  diff --git a/examples/server_async.py b/examples/server_async.py
  --- a/examples/server_async.py
  +++ b/examples/server_async.py
  @@ -313,4 +313,4 @@
   
   
   if __name__ == "__main__":
  -    asyncio.run(run_async_server("."), debug=True)
  +    asyncio.run(run_async_server(), debug=True)
  diff --git a/examples/server_sync.py b/examples/server_sync.py
  --- a/examples/server_sync.py
  +++ b/examples/server_sync.py
  @@ -313,5 +313,5 @@
   
   
   if __name__ == "__main__":
  -    server = run_sync_server(".")
  +    server = run_sync_server()
       server.shutdown()
  
  ```
  
golden_diff: diff --git a/chainer/datasets/dict_dataset.py b/chainer/datasets/dict_dataset.py
  --- a/chainer/datasets/dict_dataset.py
  +++ b/chainer/datasets/dict_dataset.py
  @@ -31,7 +31,7 @@
           batches = {key: dataset[index]
                      for key, dataset in six.iteritems(self._datasets)}
           if isinstance(index, slice):
  -            length = len(six.itervalues(batches).next())
  +            length = len(six.next(six.itervalues(batches)))
               return [{key: batch[i] for key, batch in six.iteritems(batches)}
                       for i in six.moves.range(length)]
           else:
  
verification_info: {"golden_diff": "diff --git a/chainer/datasets/dict_dataset.py b/chainer/datasets/dict_dataset.py\n--- a/chainer/datasets/dict_dataset.py\n+++ b/chainer/datasets/dict_dataset.py\n@@ -31,7 +31,7 @@\n         batches = {key: dataset[index]\n                    for key, dataset in six.iteritems(self._datasets)}\n         if isinstance(index, slice):\n-            length = len(six.itervalues(batches).next())\n+            length = len(six.next(six.itervalues(batches)))\n             return [{key: batch[i] for key, batch in six.iteritems(batches)}\n                     for i in six.moves.range(length)]\n         else:\n", "issue": "DictDataset throws an internal error only in Python 3.\nThe following code throws an internal error only in Python 3 environment with the latest version of Chainer. (3.0.0b1, 8bcac6f)\r\n```py\r\nfrom chainer.datasets import DictDataset\r\n\r\ndef main():\r\n    a = range(10)\r\n    b = range(10, 20)\r\n    dataset = DictDataset(x=a, y=b)\r\n    print(dataset[0:5])\r\n\r\nif __name__ == '__main__':\r\n    main()\r\n```\r\n\r\nIn Python 3.6.1,\r\n```sh\r\n$ python --version\r\n Python 3.6.1\r\n$ python poc.py\r\n Traceback (most recent call last):\r\n  File \"poc.py\", line 12, in <module>\r\n    main()\r\n  File \"poc.py\", line 8, in main\r\n    print(dataset[0:5])\r\n  File \"/home/igarashi/projects/chainer/chainer/datasets/dict_dataset.py\", line 34, in __getitem__\r\n    length = len(six.itervalues(batches).next())\r\nAttributeError: 'dict_valueiterator' object has no attribute 'next'\r\n```\r\n\r\nIn Python 2.7.13, \r\n```sh\r\n$ python --version\r\n Python 2.7.13\r\n$ python poc.py\r\n [{'y': 10, 'x': 0}, {'y': 11, 'x': 1}, {'y': 12, 'x': 2}, {'y': 13, 'x': 3}, {'y': 14, 'x': 4}]\r\n```\r\n\r\nIt is because an instance of `six.Iterator` doesn't have `next()` method in the Python 3 environment.\r\n[Reference](http://pythonhosted.org/six/#six.Iterator)\r\n\n", "before_files": [{"content": "import six\n\n\nclass DictDataset(object):\n\n    \"\"\"Dataset of a dictionary of datasets.\n\n    It combines multiple datasets into one dataset. Each example is represented\n    by a dictionary mapping a key to an example of the corresponding dataset.\n\n    Args:\n        datasets: Underlying datasets. The keys are used as the keys of each\n            example. All datasets must have the same length.\n\n    \"\"\"\n\n    def __init__(self, **datasets):\n        if not datasets:\n            raise ValueError('no datasets are given')\n        length = None\n        for key, dataset in six.iteritems(datasets):\n            if length is None:\n                length = len(dataset)\n            elif length != len(dataset):\n                raise ValueError(\n                    'dataset length conflicts at \"{}\"'.format(key))\n        self._datasets = datasets\n        self._length = length\n\n    def __getitem__(self, index):\n        batches = {key: dataset[index]\n                   for key, dataset in six.iteritems(self._datasets)}\n        if isinstance(index, slice):\n            length = len(six.itervalues(batches).next())\n            return [{key: batch[i] for key, batch in six.iteritems(batches)}\n                    for i in six.moves.range(length)]\n        else:\n            return batches\n\n    def __len__(self):\n        return self._length\n", "path": "chainer/datasets/dict_dataset.py"}], "after_files": [{"content": "import six\n\n\nclass DictDataset(object):\n\n    \"\"\"Dataset of a dictionary of datasets.\n\n    It combines multiple datasets into one dataset. Each example is represented\n    by a dictionary mapping a key to an example of the corresponding dataset.\n\n    Args:\n        datasets: Underlying datasets. The keys are used as the keys of each\n            example. All datasets must have the same length.\n\n    \"\"\"\n\n    def __init__(self, **datasets):\n        if not datasets:\n            raise ValueError('no datasets are given')\n        length = None\n        for key, dataset in six.iteritems(datasets):\n            if length is None:\n                length = len(dataset)\n            elif length != len(dataset):\n                raise ValueError(\n                    'dataset length conflicts at \"{}\"'.format(key))\n        self._datasets = datasets\n        self._length = length\n\n    def __getitem__(self, index):\n        batches = {key: dataset[index]\n                   for key, dataset in six.iteritems(self._datasets)}\n        if isinstance(index, slice):\n            length = len(six.next(six.itervalues(batches)))\n            return [{key: batch[i] for key, batch in six.iteritems(batches)}\n                    for i in six.moves.range(length)]\n        else:\n            return batches\n\n    def __len__(self):\n        return self._length\n", "path": "chainer/datasets/dict_dataset.py"}]}