Fix false positive in UnusedVariables for dict key subscripts

[frankentini]

Summary

Fixes #226.

Variables used as dictionary keys in subscript assignments (e.g. _var_dict[_var16] = value, generated by SETITEM opcodes) were not being recognized as "used" by unused_assignments(), causing --check-safety to incorrectly flag them as suspicious.

Root Cause

unused_assignments() iterates over statements and, for ast.Assign nodes, only records top-level ast.Name targets as definitions, then walks statement.value for used names. However, when SETITEM creates an assignment like:

_var_dict[_var16] = (_var21, 0)

The target is an ast.Subscript, not an ast.Name. The variable _var16 used as the subscript key lives inside the target and was never walked for uses -- so it appeared in defined - used despite being genuinely referenced.

Fix

When an assignment target is not a plain ast.Name (e.g. ast.Subscript, ast.Starred), walk the entire target subtree and add any ast.Name nodes found to the used set.

Test

Added regression test test_unused_variables_no_false_positive_for_dict_keys that constructs a pickle with the SETITEM dict-key pattern and verifies no false positives are reported.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[thomas-chauchefoin-tob]

@frankentini Can you please sign the CLA before we review both of your PRs? Thanks!