fix: declare the Apache-2.0 license in package metadata

[sangkyoonnam]

The wheel's METADATA has no license declaration today, which is item 3 of #901. This adds license = "Apache-2.0", so the wheel and sdist now carry License-Expression: Apache-2.0.

Changes

• pyproject.toml: add license = "Apache-2.0" next to the existing license-files.
• Raise the build requirement to flit_core >=3.11,<4. With 3.9 or 3.10 the build fails on the new field, and without it those versions also ignore license-files (a 3.10.1 build on main ships LICENSE, LICENSE-sdist and LICENSE-wheel and leaves out NOTICE and DISCLAIMER).

How I tested this

• uv build --sdist --wheel: METADATA 2.4 with License-Expression: Apache-2.0 and License-File entries for DISCLAIMER, LICENSE-wheel and NOTICE. A wheel rebuilt from the extracted sdist, as sdist-wheel-equivalence does, has the same entries.
• twine check passes for both artifacts, and scripts/verify_apache_artifacts.py artifacts passes on the wheel.
• Builds pinned to flit_core 3.11.0 and 3.12.0 succeed; 3.9.0 and 3.10.1 fail.
• pre-commit run --files pyproject.toml passes.

Notes

Release Validation installs flit without a version pin; that doesn't establish which backend built past releases. apache_release.py runs flit build, which uses the installed backend without resolving [build-system].requires, so voters on flit older than 3.11 need pip install --upgrade flit.

Other items from #901:

• 2 (sdist side) and 4: since Create sdist license #899 the release helper installs LICENSE-sdist as the release sdist's LICENSE (a plain uv build doesn't). The wheel still ships licenses/LICENSE-wheel, and verify_apache_artifacts.py requires that name.
• 5: license-files points at LICENSE-wheel, and sdist-wheel-equivalence builds the wheel from the sdist, so dropping it from the sdist would break that job unless the wheel's license file comes from somewhere else.
• 1: 0.43.0.incubating isn't a valid PEP 440 version, so that exact filename won't work. A wheel build tag could carry it while keeping version 0.43.0, e.g. apache_burr-0.43.0-1incubating-py3-none-any.whl. Build tags must start with a digit and change which wheel pip prefers among otherwise equal ones; flit doesn't produce them today, and I haven't checked PyPI acceptance.

This PR only covers item 3. Items 2 (wheel side) and 5 need a decision on how the wheel gets its license file, and item 1 a naming decision. Happy to follow up on whichever direction you prefer.

I wrote this with Claude Code and reviewed and ran it myself.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.