Add B043: Do not call delattr with constant

[fennr]

close #513

I used B043 because I see PR with B042

[fennr]

@cooperlees check my PR please

[Copilot]

Pull Request Overview

This PR adds a new linting rule B043 that detects when delattr() is called with a constant string attribute name, suggesting to use direct attribute deletion instead.

• Adds rule B043 to detect delattr(obj, "constant") usage patterns
• Provides test cases demonstrating valid and invalid usage patterns
• Updates documentation to explain the rule and its rationale

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/eval_files/b043.py	Test cases showing valid dynamic usage vs invalid constant usage of delattr
bugbear.py	Core logic to detect delattr calls with constant string arguments and error definition
README.rst	Documentation explaining B043 rule and recommending del x.attr over delattr(x, 'attr')

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