Use tomllib from the standard library on Python 3.11+

[hroncok]

Hey,
would you accept a pull request that uses tomllib rom the standard library on Python 3.11+ and fallbacks to tomli, instead of toml?

[hroncok]

I was testing the changes, so I opened a draft PR without a changelog entry to present what change I meant. If acceptable, I'll add the entry. #2463

[jugmac00]

Looks like tomli never had support for Python 2.7. I am not sure what are the plans about that in the future. tox4 will require Python 3.7 as a minimum, but I don't think it makes a lot of sense to drop support for Python 2.7 for tox 3 anymore.

[hroncok]

I have not thought about that. Will fallback to toml. Not today though.

[hroncok]

Done.