400 bad request using Python 3 (mediatools or discisrc backend)

Using Python 3.3.2 (32 bit) on Windows 7 submission fails:
```
found new ISRC for track 1: b'GB'b'BBN'b'79'b'02002'
...
ERROR: Couldn't send ISRCs: caused by: HTTP Error 400: Bad Request
```

I want to note that using Python 2.7 it works on Windows (7) and using Python 3.3.2 it works on Linux.