B031: allow reusing groups after materialization

[Sonike]

After group = list(group), B031 currently flags later reads of group even though it now holds a reusable list. This recognizes explicit list(group) and tuple(group) assignments back to the same name, including annotated and chained assignments.

The assignment's right-hand side is checked before the name becomes reusable. Conditional paths are merged conservatively, so a conversion on only one branch or inside a potentially empty nested loop does not suppress genuine repeated-use warnings. Arbitrary calls such as iter(group) retain the existing behavior; this does not add alias analysis or infer the return types of helper functions.

Fixes #395.

Validation:

• Confirmed that the initial regression cases fail on the current main branch, reporting three spurious B031 diagnostics, and pass with this change.
• tox -e py313,py314: Python 3.13 — 80 passed, 1 version-specific skip; Python 3.14 — 81 passed. Both report 98% coverage.
• pre-commit run --all-files: isort, black, flake8, and rstcheck passed.
• Regression coverage includes direct/annotated/chained assignments, both and partial conditional branches, previous consumption, assignment to another name, iter(), nested loops, and deferred function bodies.

[Copilot]

🟢 Approval recommended

The implementation and new eval coverage align with the stated behavior change and appear to preserve conservative warnings in ambiguous control-flow cases.

Pull request overview

This PR updates the B031 check (groupby-group reuse) so that reassigning group = list(group) or group = tuple(group) back to the same variable is recognized as “materialized and now reusable,” avoiding false positives like #395 while still being conservative in conditionals and potentially-empty nested loops.

Changes:

• Teach B031’s AST walk to detect same-name list(...) / tuple(...) materialization assignments (including annotated and chained assignments) and treat the name as reusable afterward.
• Adjust B031 path-merging for nested loops and other constructs to avoid assuming assignments in potentially-optional/deferred execution definitely ran.
• Add eval coverage for direct/annotated/chained materialization, conditional branches, nested loops, iter(), and deferred function bodies; document the change in the changelog.

File summaries

File	Description
bugbear.py	Adds materialization detection for B031 and refines control-flow handling to avoid spurious repeated-use warnings after list(group) / tuple(group) reassignment.
tests/eval_files/b031.py	Adds regression/eval cases covering materialization and conservative merging scenarios (conditionals, nested loops, deferred bodies).
README.rst	Notes the B031 behavior change in the UNRELEASED changelog.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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[cooperlees]

Agree with copilot. Many thanks for adding this!

[cooperlees]

@copilot resolve the merge conflicts on this branch.