Make the TOML loaded to use `tomllib` with Python ≥3.11 by default

[frank-lenormand]

The toml library used by confight still doesn’t fully comply with the TOML v1.0.0 specification, and seems abandoned.

Example of problematic shortcoming (several years old) that still isn’t fixed:

• loading of mixed typed arrays
• defined at: https://toml.io/en/v1.0.0#array

Python v3.11 has introduced a library for TOML parsing (read-only): tomllib.

It’s not clear if using tomllib over toml would be a net benefit (trading a set of bugs for another is not ideal).

There’s a good chance that using tomllib by default when Python ≥3.11 is used as an interpreter will be beneficial if not now, at least long-term.