tox: stop py3-unit re-pinning to Python 3.11, let the CI matrix actually run

[UgaTheDev]

unit.yaml runs tox -e py3-unit across a matrix.python of 3.11, 3.12,
and 3.13 (three separate jobs, each with actions/setup-python installing
the matching interpreter). But [testenv:py3-unit] in tox.ini sets
basepython = python3.11, so tox re-pins every leg back to 3.11 regardless
of which interpreter setup-python put on PATH. All three matrix legs
currently exercise the same interpreter — the 3.12/3.13 legs are decorative.

This removes that pin. With no basepython override, tox's uv-venv-runner
uses the interpreter already on PATH for that job — i.e. whatever
actions/setup-python set up for the current matrix leg — so the three CI
jobs actually diverge.

Scope is intentionally narrow: only py3-unit. py3-smoke (tox.ini line 53)
keeps its basepython = python3.11 pin — it isn't part of the version matrix
in smoke.yaml/smoke-py312.yaml and shouldn't silently start floating.

Verified locally: tox -l still lists py3-unit with the same commands;
only the interpreter selection changes. No other env is touched.

Originally flagged in #705 (comment from 2026-07-29): "the 3.11/3.12/3.13 matrix is decorative: [testenv:py3-unit] in tox.ini sets basepython = python3.11, so tox re-pins the interpreter to 3.11 on all three legs no matter what setup-python installed."

Fixes the tox side of the CI-matrix finding in #705.

Summary by CodeRabbit

• Chores
  • Updated the Python 3 unit-test environment to use tox’s default interpreter selection.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 50a42213-f3b2-4d65-80ba-d7815450ad7f

📥 Commits

Reviewing files that changed from the base of the PR and between da76404 and d1123df.

📒 Files selected for processing (1)

• tox.ini

💤 Files with no reviewable changes (1)

• tox.ini

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📝 Walkthrough

Walkthrough

The py3-unit tox environment no longer sets basepython to Python 3.11. Tox now selects the interpreter by default.

Changes

Tox interpreter selection

Layer / File(s)	Summary
Use tox default interpreter selection tox.ini	The py3-unit environment no longer explicitly selects Python 3.11. Tox uses its default interpreter selection.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to d1123

The change allows each py3-unit CI matrix job to use its configured Python version while leaving other environments unchanged; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: removing the Python 3.11 pin so the CI matrix can use each configured Python version.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[UgaTheDev]

All four of these (#712, #713, #714, #715) are mergeable with zero human reviews — and CodeRabbit rate-limited on three of them, so they've had no review at all. Since the constraint looks like review bandwidth rather than the changes themselves: want me to consolidate the four into a single PR so it's one review pass instead of four? They're independent commits, so either shape works — say the word and I'll have it up the same day.