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.

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1. https://github.com/ComPWA/ampform-dpd
2. MAINT: update lock files ampform#439
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3. https://github.com/ComPWA/benchmarks
4. https://github.com/ComPWA/compwa.github.io
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6. https://github.com/ComPWA/K-matrix-research/pull/97
7. https://github.com/ComPWA/Nstar-docs
8. https://github.com/ComPWA/polarimetry
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11. https://github.com/ComPWA/PWA-pages
12. MAINT: update lock files qrules#289
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13. https://github.com/ComPWA/report
14. https://github.com/ComPWA/sphinx-hep-pdgref
15. https://github.com/ComPWA/strong2020-salamanca
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18. MAINT: update lock files redeboer/bossdoc#62
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