Use SPDX string form for the license field in pyproject.toml

[elhoim]

Finding 31 (Medium) — pyproject.toml:11,2

Problem

The PEP 621 license-table form is deprecated upstream while build-system requires setuptools>=68 with no ceiling and the workflow installs the newest build/setuptools each run, so a future setuptools release can turn today's warning into a hard build failure with no change in this repo.

Fix

pyproject.toml used the deprecated PEP 621 file-table form for project.license ({ file = "LICENSE" }). Since build-system.requires pins setuptools>=68 with no upper bound and CI installs the latest setuptools/build each run, a future setuptools release could turn today's deprecation warning into a hard build failure. Fixed by switching to the SPDX license expression string form: license = "AGPL-3.0-or-later", which matches the AGPLv3 text already in LICENSE.

Verification

Reproduced against the unmodified code at 9b8c605, then re-checked after the change.

Before

Invoked setuptools.build_meta.prepare_metadata_for_build_wheel() directly on a copy of the pyproject.toml with `license = { file = "LICENSE" }`:
WARN: SetuptoolsDeprecationWarning `project.license` as a TOML table is deprecated
!!
        ********************************************************************************
        Please use a simple string containing a SPDX expres[sion]...

After

Same call with `license = "AGPL-3.0-or-later"`: dist_info builds cleanly with no deprecation warning emitted.

python bin/cli.py --help exits 0 and the module still imports cleanly. Verification was performed offline against the pure functions — no running misp-modules instance is required.

Branched from 9b8c605. This PR addresses only this finding; the other findings from the same review are in separate PRs, so they will need rebasing against each other as they merge.

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