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

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

Is This a Code Change?

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

Validation

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

[CLAassistant]

All committers have signed the CLA.

[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 :-)

[hugovk]

This PR has a merge conflict.

[a-detiste]

This PR has a merge conflict.

fixed

[AndreyVMarkelov]

Thanks for this, @a-detiste — and apologies it sat open so long. The six removal has now landed on main via #365, which dropped six from requirements.txt and replaced all usages across the obj_c, swift, and python_rsrc backends (using an ensure_str helper to preserve the bytes-decoding semantics), simplified _strftime, and modernized the Python floor/CI along the way. main now has zero six references.

Your PR informed that direction, so thank you for it. Since the goal is fully covered on main and this branch is now several commits behind (and would conflict), I'm closing this as superseded by #365. Really appreciate the contribution!