fix: read config data with bytes (python3)

[tomplus]

I came across a problem where kubeconfig has certificates stored as binary data.

apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: !!binary |
      TAMwdExTSUNSVWRKVGlCRFJWOlVTVVpKUTBLVVJTMHRMUzB0A2sxSlNVWm1SRU5EUVRKVFowRjNT
      ............
      bZBKUkhZeFIzQnNARGRuUFQwS0xTMHFMUzFGVGtRZ1EwVlNWRWWHU1VOQlZFZXRMUzB1TFE9PQ==
...

Python 3 sees this as bytes and the encoding problem appears.

Traceback (most recent call last):
  File "examples/example1.py", line 33, in <module>
    main()
  File "examples/example1.py", line 22, in main
    config.load_kube_config()
  File "/tmp/python/kubernetes/config/kube_config.py", line 520, in load_kube_config
    loader.load_and_set(config)
  File "/tmp/python/kubernetes/config/kube_config.py", line 402, in load_and_set
    self._load_cluster_info()
  File "/tmp/python/kubernetes/config/kube_config.py", line 381, in _load_cluster_info
    file_base_path=self._config_base_path).as_file()
  File "/tmp/python/kubernetes/config/kube_config.py", line 104, in as_file
    base64.decodestring(self._data.encode()))
AttributeError: 'bytes' object has no attribute 'encode'

In this PR I've added a check if _data are the str type.

Thanks.

[roycaihw]

nit: you can use TEST_DATA.encode() or bytes(TEST_DATA, encoding='utf-8') instead of introducing new variables

[tomplus]

+1, fixed

[roycaihw]

Thanks! Could you squash the commits?

[tomplus]

Done

[tomplus]

/test all

[tomplus]

I have no idea how to test it again. The previous build failed but it looks like an Travis' issue.

[roycaihw]

/lgtm
/approve

Tests are green now. I manually re-triggered the tests. It would be great if we have the retest commends. Maybe worth a test-infra issue...

[k8s-ci-robot]

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