DeprecationWarning from kube_config.py

Most folk won't see these as they default hidden in widely used Python versions.

```
17:24:57 ./.tox/venv/lib/python3.5/site-packages/kubernetes/config/kube_config.py:102: DeprecationWarning: decodestring() is a deprecated alias, use decodebytes()
17:24:57    base64.decodestring(self._data.encode()))
```

Using `base64.standard_b64decode` will avoid this.
