Support Metadata 2.4 and 2.5, update vendored 'packaging' to 26.0

[jameslamb]

In response to loud deprecation warnings from setuptools, RAPIDS projects recently all switched to PEP 639 license metadata, which bumps their wheel Metadata-Version to 2.4 (ref: rapidsai/build-planning#152).

Stubs currently can't be built from those wheels, because this project does not support wheel Metadata-Version beyond 2.3.

This fixes that, with the following:

• updates vendored packaging from 24.1 to 26.0
• modifies assert_valid_metadata() to allow for wheels that have License-Expression but not License (License was deprecated beginning with Metadata-Version 2.4)
• adds docs on how to update the vendored code

Notes for Reviewers

How I updated packaging

0 manual changes were made in _vendor/. Updated packaging to 26.0 (its latest version) like this:

pushd ./src/wheel_stub/_vendor
git rm -rf packaging*

pip download --no-deps 'packaging==26.0'
unzip \
    -o \
    packaging*.whl

rm *.whl

popd

How I tested this

Followed the instructions in the README, using a wheel I know this project failed to produce an sdist for.

testing code (example)

# clean up from previous attempts
rm -rf ./dist
rm -rf ./demo/dist
rm -f ./demo/*.whl
rm -rf ./.venv

# work in a venv
python -m venv .venv
source .venv/bin/activate

# install build dependencies
pip install hatch hatchling

# build wheel-stub
hatch build -t wheel

# try to build an sdist from a cuCIM 26.2 wheel
pushd demo
WHEEL_URL="https://pypi.anaconda.org/rapidsai-wheels-nightly/simple/cucim-cu13/26.2.0a35/cucim_cu13-26.2.0a35-cp313-cp313-manylinux_2_27_x86_64.manylinux_2_28_x86_64.whl"

wget "${WHEEL_URL}"

PIP_FIND_LINKS=$(pwd)/../dist \
PIP_NO_INDEX=1 \
   python -m build \
      --sdist \
      --config-setting source_wheel=cucim_cu13-26.2.0a35-cp313-cp313-manylinux_2_27_x86_64.manylinux_2_28_x86_64.whl

On main, this fails in the same way we observed in internal pipelines building a wheel-stub sdist:

Traceback (most recent call last):
  File "/home/jlamb/repos/wheel-stub/.venv/lib/python3.12/site-packages/pyproject_hooks/_in_process/_in_process.py", line 389, in <module>
    main()
...
  File "/tmp/build-env-pho0yp4y/lib/python3.12/site-packages/wheel_stub/sdist.py", line 148, in build
    self.assert_valid_metadata(parsed_metadata)
  File "/tmp/build-env-pho0yp4y/lib/python3.12/site-packages/wheel_stub/sdist.py", line 224, in assert_valid_metadata
    assert "License" in parsed, "Software should have a license"
           ^^^^^^^^^^^^^^^^^^^
AssertionError: Software should have a license

ERROR Backend subprocess exited when trying to invoke build_sdist

On this branch, it succeeds

* Creating isolated environment: venv+pip...
* Installing packages in isolated environment:
  - wheel-stub
* Getting build dependencies for sdist...
* Building sdist...
Successfully built cucim_cu13-26.2.0a35.tar.gz

Also ran all the tests

$ pip install -r ./requirements/requirements-lock.txt .
$ pytest
=== 11 passed in 13.94s ===

[jameslamb]

This file is the only Python code I manually changed, just calling it out so it isn't lost in the diff.

At https://packaging.python.org/en/latest/specifications/core-metadata/#license it says:

License
Deprecated since version 2.4: in favour of License-Expression.