Release Zarr-Python v3.4.0 #4256

Release: v3.4.0 (originally planned as v3.3.1; renamed because main now carries two features — #4063 adds a required msgspec dependency and stricter metadata parsing, and #4277 changes the exception types raised for unknown codecs)
Scheduled Date: 2026/08/13

Priority PRs/issues to complete prior to release

• "ValueError: Input array must be contiguous" when *cast_value* codec next to *transpose* codec #4237
• fix(chunk-grids): make array creation O(1) in per-dimension chunk count #4218
• The roadmap: docs: add roadmap page outlining future plans #4149
• Add unified JSON metadata validation via msgspec (#3285) #4063

Before release:

• Check SPEC 0 to see if the minimum supported version of Python or NumPy needs bumping.
• Verify that the latest CI workflows on main are passing: Tests, GPU Tests, Hypothesis, Docs, Lint, Wheels.
• Run the downstream tests against main (passed 2026-09-09: https://github.com/zarr-developers/zarr-python/actions/runs/34338567820; re-run on 3203018c after 12 more merges: https://github.com/zarr-developers/zarr-python/actions/runs/34414991933): go to the workflow page, click "Run workflow", and select the main branch. Verify that the Xarray and numcodecs integration tests pass.
• Open a release PR with the changelog entries for the upcoming release (docs: 3.4.0 release notes #4360, draft), generated with uv run --only-group release towncrier build --version x.y.z.
• Review the release PR and verify the changelog in docs/release-notes.md looks correct.
• Merge the release PR.

Release:

• Draft a new GitHub Release with tag vX.Y.Z targeting main. Use "Generate release notes" for the description.
• Verify the release is published on PyPI and ReadTheDocs.

After release:

• Review and merge the pull request on the conda-forge zarr-feedstock that will be automatically generated. Note: this release adds msgspec>=0.19 as a required dependency, so the recipe needs a manual edit.

• Party 🎉