Fix Python 3 bytes-string Error

Since Python 3 switches the base string class, now calling the Python 2 str class bytes, an error was being thrown due to implicit conversion:

```
File ".../apns.py", line 288, in _get_notification
    + payload_length_bin + payload_json)
TypeError: Can't convert 'bytes' object to str implicitly
```

This update corrects this, checking which version of Python is running.