Use the modern tomli/tomllib to parse TOML files

[mgorny]

Replace the unmaintained toml package with the modern combination of built-in tomllib in Python 3.11+, and its drop-in replacement tomli for older Python versions. This provides full and correct support for TOML 1.0 that toml does not provide. Unfortunately, they do not support overriding dict class but that does not seem to be a major problem, given that TOML tables are not ordered according to the specification.

[oz123]

I would be very happy if this is merged, as it will allow us to drop toml from the vendored dependencies we ship to our pipenv users.

[mgorny]

Thank you!