[keyvault] Remove Python 3.6 and six references

[wonhyeongseo]

Description

Dear @mccoyp, after review we found some changes were lost during updates.
Thank you.

Is your feature request related to a problem? Please describe.
In accordance with the August Milestone of Azure for dropping Python 3.6 support, changes that were made to the keyvault packages have been recovered. We also recovered @ponopono0322's contribution of removing references to the six package.

Describe the solution you'd like
Drop Python 3.6 support and add 3.10 to the supported versions list.
Remove all references of six package from keyvault code.

Describe alternatives you've considered
None

Additional context
#25449
#25902

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[ghost]

Thank you for your contribution wonhyeongseo! We will review the pull request and get back to you soon.

[mccoyp]

Thank you so much again for doing this! Dropping six is a lot of work, and I really appreciate it 🙂 I'm sorry about the late review on this; there have been a number of releases in KV recently and it was difficult to merge things in while that was being worked on.

Based on your PR description, it sounds like we can close #25902 and use this PR instead. Is that correct?

[mccoyp]

By the way -- I'm currently regenerating our generated code, and that will drop the implicit six dependency and should get CI passing on this PR. I'll merge the regeneration work soon and then we can merge this right after 🙂

[mccoyp]

Hi again @wonhyeongseo, I'm back! The code regeneration I mentioned earlier just went through today: #27170

If you update this PR with main, this PR should be just about ready to go. There are a number of merge conflicts since this has been up for a while though (which is because of us -- my apologies), so it may be easier to open a separate PR with these changes.

One last note: the azure-mgmt-keyvault package is owned by a different set of folks than the azure-keyvault-* packages, so it would be best to leave azure-mgmt-keyvault changes for a separate PR. You can tag me if you'd like to open one, and I would make sure the package owners see it!

Thank you again for the contribution and for your patience!

[wonhyeongseo]

Closing in favor of #27377