RFE: Prefer TOML-native configuration over legaci INI

[hroncok]

What's the problem this feature will solve?

• I want old tox versions to know I need tox >= 4.21
• I want to use TOML-native configuration

If I have this:

[tool.tox]
requires = ["tox>=4.21"]
env_list = ["3.13", "3.12", "type", "doc"]
...

The old tox does not read it and ignores it.

If I change it like this:

[tool.tox]
legacy_tox_ini = """
    [tox]
    min_version = 4.21
"""
env_list = ["3.13", "3.12", "type", "doc"]
...

The old tox reads it and updates tox in the provision venv.
However, the TOML-native configuration is ignored by the new tox.

Describe the solution you'd like

I believe this could be solved if the new tox preferred the TOML-native configuration over legacy_tox_ini if both exist.

Alternative Solutions

I could stick with legacy_tox_ini unless I know all environments are fully updated to tox > 4.21.

Thanks

[gaborbernat]

PR welcome 👍 likely just need to swap it here. https://github.com/tox-dev/tox/blob/2bf065a/src/tox/config/source/discover.py#L22 and add test to confirm it 👍 plus updating the docs.

[hroncok]

I think this is useful, so I'll try to dedicate some time to work on this this week, but no promises.