This came out in #19115

Recent tox versions gradually introduced a new configuration format: tox.toml
It is now officially advertised as

• supporting every features from tox.ini (albeit with a different style)
• the recommended format
• the only target for future features (well, I guess that would actually imply [tool.tox] would also be viable)

I think this means we need to migrate eventually: sooner or later, we might need a feature that only exists in this new format, and it's also plausible that bugs might not get fixed as promptly when they affect a legacy format.
Exceptionally, I'll assign myself to this ticket for a simple reason: I already started doing the work, initially as an experiment, but now I think it's necessary and will create a PR once I'm there.

Linked issues and PRs

• ENH: allow tox.toml in rule PY007 scientific-python/cookie#739
• list_dependencies_command doesn't seem to work with tox.toml tox-dev/tox#3830