New GPT2 tokenizer no longer encodes Unicode characters properly in Python 3
In commit 5afa497cbfc53c679a9b22997b6312fad57ee2f8, you changed `token.encode('utf-8')` to simply `token`.

This would make the code compatible with Python 2, but now it breaks in Python 3. You'll get a KeyError when you try to encode a Unicode character that requires more than 1 byte in UTF-8 encoding. For example, this raises a KeyError in Python 3:

```python
from pytorch_pretrained_bert.tokenization_gpt2 import GPT2Tokenizer
tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
tokenizer.encode('你')
```

I think what you want to do is:

```python
if sys.version_info[0] == 2:
    token = ''.join(self.byte_encoder[ord(b)] for b in token)
else:
    token = ''.join(self.byte_encoder[b] for b in token.encode('utf-8'))
```
Just ran into this problem. This seems to be a regression from an earlier version of Huggingface.

For instance it fails when encoding the following wikipedia snippet
> The dismemberment of the French socialist movement into many groups and—following the suppression

The dash here is "long dash" with unicode 8212. This worked in earlier version because it worked on bytes.<|||||><img width="992" alt="image" src="https://user-images.githubusercontent.com/44499264/59059983-a5579180-88d2-11e9-9124-f7ce32f20419.png">
I can confirm that this is happening, though it is a different dash.<|||||>> <img alt="image" width="992" src="https://user-images.githubusercontent.com/44499264/59059983-a5579180-88d2-11e9-9124-f7ce32f20419.png">
> 
> I can confirm that this is happening, though it is a different dash.

Same here:
This is also happening while using GPT2 tokenizer:

`
Traceback (most recent call last):
  File "run_lambada_gpt2.py", line 139, in tokenize_and_encode
    token_ids = tokenizer.encode(obj)
  File "/data/anaconda/envs/py35/lib/python3.5/site-packages/pytorch_pretrained_bert/tokenization_gpt2.py", line 261, in encode
    return self.convert_tokens_to_ids(self.tokenize(text))
  File "/data/anaconda/envs/py35/lib/python3.5/site-packages/pytorch_pretrained_bert/tokenization_gpt2.py", line 224, in tokenize
    token = ''.join(self.byte_encoder[ord(b)] for b in token)
  File "/data/anaconda/envs/py35/lib/python3.5/site-packages/pytorch_pretrained_bert/tokenization_gpt2.py", line 224, in <genexpr>
    token = ''.join(self.byte_encoder[ord(b)] for b in token)
KeyError: 8217
`


The sys version info is:
`
sys.version_info(major=3, minor=5, micro=5, releaselevel='final', serial=0)
`
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>Hi,
I'm about to use this tokenizer with python3 on wiki-text.
After seeing this issue - I'm not sure if it will work properly.

Can someone clarify please? 
From reading along seems like the fix suggested above did not solve the problem, right?
<|||||>Hi, this looks fixed to me in the current implementation. As long as you're using a recent version of the library you should be fine. I had no problem running a fine-tuning script on wikitext-2 last week.

If you run into anything, please let me know and I'll look into it.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
