python: Remove usage of six

[akx]

This PR removes remaining traces of the six Python 2/Python 3 compatibility library; the Python generator targets Python 3.7+ only, so it isn't needed for anything.

The idea is to eventually drop the six dep from https://github.com/kubernetes-client/python too.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples (...)
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
  • This should not break anything, since the generators are targeting Python 3.7 anyway.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  • 👋 @krjakbrjak