Prefer tox.toml over tox.ini if both exist

[webknjaz]

So during migration of my repos to a new laptop I accidentally ended up with a tox repo checkout having an old tox.ini on disk. When I was trying out tox r, it was yelling at me about some CLI args that pytest didn't recognize.
It took me some time to see that tox l outputs envs from tox.ini and not tox.toml.

Action items:

• Decide whether to warn or error out if two definitions exist
• Make the TOML definition take precedence

[gaborbernat]

duplicate issue of #3402