remove dependency on six

[a-detiste]

Your project states "Stone requires Python 3.", so six can be dropped.

https://wiki.debian.org/Python3-six-removal

Checklist

General Contributing

• [x] Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• [ ] Non-code related change (markdown/git settings etc)
• [x] Code Change
• [ ] Example/Test Code Change

Validation

• [x] Have you ran tox?
• [?] Do the tests pass?

[CLAassistant]

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[greg-db]

Thanks for putting this together! I'll ask the team to review this.

[a-detiste]

It'more about tearing appart than putting together :-)