Add liccheck to gate CI on dependency license compliance

[janhoy]

Summary

Add liccheck with an authorised-license
allowlist in setup.cfg so that CI fails if any (transitive) Python dependency carries an
unapproved license (e.g. GPL, LGPL, or ASF Category X licenses).

Motivation

• Apache Superset uses liccheck to automatically flag problematic transitive deps
• Known issue: certifi is MPL-2.0 (ASF Category B — allowed with attribution, not
  bundled); liccheck will surface this and similar cases
• Avoids manual license audits when adding or upgrading dependencies

Proposed changes

1. Add [liccheck] section to setup.cfg (or a liccheck.ini) with:
   • authorized_licenses allowlist (Apache-2.0, MIT, BSD-*, ISC, Python-2.0, …)
   • unauthorized_licenses blocklist (GPL, LGPL, AGPL, …)
2. Add liccheck to dev/CI dependencies
3. Add make liccheck target
4. Integrate into CI pipeline (fail on unapproved licenses)
5. Document in DEVELOPER_GUIDE.md

References

• https://github.com/dhatim/python-liccheck