chore(deps): remove tornado dependency and clean up references

[LaikaN57]

Summary

Final PR in the tornado removal migration. Remove the tornado==6.5.4 dependency and clean up remaining references.

What changed

• pyproject.toml: Removed tornado==6.5.4 and its comments from [project] dependencies
• uv.lock: Regenerated without tornado (confirmed: Removed tornado v6.5.4)
• README.rst: Changed "Kingpin leverages python's tornado engine" to "Kingpin leverages python's asyncio library"
• actors/group.py: Removed stale comment referencing "tornado-ism"

What was NOT changed (intentional)

• mock_tornado() and tornado_value() in actors/test/helper.py — pure-Python helpers with no tornado imports. Renaming would touch ~100 call sites for zero behavioral benefit (documented in retro.md).

Migration summary (PRs 1-10)

Metric	Value
PRs merged	10
Files changed	~25
Runtime deps removed	1 (tornado==6.5.4)
Runtime deps added	0
Tests before	361
Tests after	360 (1 deleted: tested tornado-specific return type)

Why this is safe

• All tornado imports were removed in PRs 1-9 — this PR only removes the dependency declaration
• If any code still imported tornado, make test would fail with ModuleNotFoundError
• kingpin --help verified working without tornado installed

Test plan

• make test passes (360 tests, 0 failures)
• kingpin --help works without tornado
• grep -rn "tornado" kingpin/ --include="*.py" returns zero results (excluding helper function names)
• ruff check clean

Part 10 of 10 — tornado removal complete! 🎉

🤖 Generated with Claude Code