release: bump version to 4.3.0

[sbneto]

TL;DR

• bump-my-version bump minor: 4.2.0 → 4.3.0 (pyproject + __version__), ahead of the develop → master release PR that carries the 4.3.0 changelog.

[claude]

Reviewed against AGENTS.md §Gitflow and specs/05-downstream-contract.md §Versioning/§Release flow — clean, no action needed.

Checks made:

• Base is develop, and a version bump as a prep commit on develop just before the develop → master merge is explicitly sanctioned (specs/05-downstream-contract.md:410: "The version bump goes in this PR (or as a prep commit on develop just before the merge)"). Pairs with Release 4.3.0 #319 (develop → master, carries the 4.3.0 changelog), which currently has no competing version edit — its pyproject.toml delta is the pytest-opts change from test: quieter pytest output for the live e2e run #315, so this PR is the sole source of the bump.
• Minor is the right size. Everything on develop since 4.2.0 is feat: typed exception for a withheld known-good binary #316 (KnownGoodWithheldException, which subclasses NotFoundException — exceptions.py:67), fix: restore pre-4.0 204 ("no results") handling for exists() and downloads #313/test: quieter pytest output for the live e2e run #315/test: use the current triage VM slug, not the deprecated win10 one #317 (fix + test-only). Per the table KnownGoodWithheldException is both "new exception class that subclasses an existing one" and "narrowing which exception a status maps to" → minor, not major; nothing removes or re-signs a public symbol.
• Both version sites move together and match [tool.bumpversion]: pyproject.toml version + current_version, and src/polyswarm_api/__init__.py __version__ (the single configured extra file, pyproject.toml:73-76). No stale 4.2.0 left anywhere else in the tree — settings.py reads polyswarm_api.__version__ rather than pinning a literal.
• Commit is a single tool-generated bump-my-version commit, no AI-attribution trailer, no ticket IDs or private repo names.