Stop using deprecated urllib3 API

[arcivanov]

What happened (please include outputs or screenshots):

/home/arcivanov/.config/kubernator/python/20/kubernetes/client/exceptions.py:91: DeprecationWarning: HTTPResponse.getheaders() is deprecated and will be removed in urllib3 v2.1.0. Instead access HTTPResponse.headers directly.
  self.headers = http_resp.getheaders()

What you expected to happen:
No deprecation warning

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Kubernetes version (kubectl version):
• OS (e.g., MacOS 10.13.6):
• Python version (python --version)
• Python client version (pip list | grep kubernetes)