removed six dependency

[inc-ali]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

It removes the need to the six dependency, which was mostly present in the generated openapi client code. Python 2 support was dropped a long time ago.

It was previously tried but abandoned in #2224

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

The changes to the generated classes are simple, please focus on changes to ws_client.py.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[k8s-ci-robot]

PR needs rebase.

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