Name 'unicode' is not defined 

Hi,

I was using `loadRes` in `ytvos.py` at [Line 216](https://github.com/youtubevos/cocoapi/blob/master/PythonAPI/pycocotools/ytvos.py#L216), and I came across the following error:
`NameError: name 'unicode' is not defined`

I believe it has something to with Python3 replacing unicode with str.

Is pycocotools not built for Python3? Could this be changed?