fix: clean up packaging metadata

[berisher]

Summary

• fix the remaining publish-blocking package metadata and install docs for the crates.io distribution
• align the documented Python prerequisite with the project’s 3.11 minor-version support policy
• keep install-time attestation enforcement deferred until there is a new attested release to consume

Remediated Hunter findings

• F-0054 Cargo.toml declared six keywords, which crates.io rejects
• F-0056 README.md told users to install an unowned crate name
• F-0062 pyproject.toml had already been relaxed, but DEVELOPING.md still documented a single CPython micro-release

Notes

• F-0057 is the analogous install-string issue in bench_hide/blog.md, but that file is an ignored local draft (bench_*/ in .gitignore), not a tracked repo file, so it is not part of this PR
• the deferred attestation follow-up remains unchanged: once a new attested release exists, restore strict gh attestation verify enforcement in the install path

Changes

• trim the published crate keywords to the five-keyword crates.io limit
• switch the README crates.io install command to cargo install cisco-ai-defense-pickle-fuzzer
• document that the package installs the pickle-fuzzer binary
• update DEVELOPING.md to require Python 3.11.x instead of a single micro-release

How to validate

• cargo package --allow-dirty --no-verify
• cargo test
• git diff --check