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I'm personally not super keen on this (yet) - the reason is that if we do this we really should do it across all astropy packages for consistency, and there's something to be said for being familiar with a format. The tox.ini file is still the first one that is checked (https://tox.wiki/en/latest/reference/config.html#discovery-and-file-types). I would personally advocate waiting until the tox project recommend that existing projects migrate over to it and/or deprecate tox.ini, as it only recommends it for new projects at the moment – unless there's actually a benefit to switching over given the cost of migrating all the packages in the astropy ecosystem. At the moment, I find the TOML file a lot harder to read/parse visually, so it's not clear to me what the benefit of upgrading is.

if we do this we really should do it across all astropy packages for consistency

definitely.

I find the TOML file a lot harder to read/parse visually

both are really hard to me. At least TOML is a well established format that is used elsewhere, so I'm still seeing a gain here:

• reading it is a re-usable skill (while tox.ini is extremely tool specific)
• it may put a bit of pressure on tox development to avoid making the configuration even more complex in the future (hopefully)

Though in my opinion the bulk of the benefit comes from trying to migrate now, so we get a chance to report problems upstream before the dust has settled and we're stuck between an unsupported format and something that doesn't work for us.

Ok sounds good - I have no objections with having this open as an experiment (I've put the relevant label) to see if it's possible to get it working and what the final thing would look like, but I think we'll need to have a broader conversation if we want to go from experiment to actually merging this.