Merge tox.ini into pyproject.toml with legacy method

[redeboer]

As a step towards #180, the tox.ini file could already be merged into pyproject.toml using tool.tox.legacy_tox_ini. The disadvantage is that there is no proper INI syntax highlighting when using a pure string in a TOML file, but then again, INI has no absolute standards for syntax highlighting (see e.g. the tox-specific string substitution). Also note that with #408, the configuration should be significantly smaller and therefore easier to maintain even without syntax highlight.

Pull Requests

Preview Give feedback

1. https://github.com/ComPWA/ampform-dpd
2. DX: define developer environment with uv ampform#439
   10 of 10
   🔨 Maintenance 🖱️ DX +2
   
3. https://github.com/ComPWA/benchmarks
4. https://github.com/ComPWA/compwa.github.io
5. https://github.com/ComPWA/demo
6. https://github.com/ComPWA/K-matrix-research/pull/97
7. https://github.com/ComPWA/Nstar-docs
8. https://github.com/ComPWA/polarimetry
9. https://github.com/ComPWA/policy
10. https://github.com/ComPWA/PWA-JPsi2pbarSigmaKS
11. https://github.com/ComPWA/PWA-pages
12. DX: define developer environment with uv qrules#289
    🔨 Maintenance 🖱️ DX +2
    
13. https://github.com/ComPWA/report
14. https://github.com/ComPWA/sphinx-hep-pdgref
15. https://github.com/ComPWA/strong2020-salamanca
16. DX: define developer environment with uv tensorwaves#533
    10 of 10
    🔨 Maintenance 🖱️ DX +2
    
17. https://github.com/ComPWA/update-pip-constraints
18. DX: define developer environment with uv redeboer/bossdoc#62
    🔨 Maintenance 🖱️ DX +2