Use a different TOML package

[riley-harper]

The TOML parser that we're using appears to be unmaintained, or at least not regularly updated. It has some known bugs that keep it from matching the TOML standard, and some of the error messages are not particularly helpful. The tomli crate is a newer crate that will be added into the standard library in Python 3.11 as tomllib. Let's move to using tomli instead of toml. I believe some of our dependencies are already using tomli, so this will actually reduce the number of packages installed in the environment.

This may change how hlink handles some configuration files, especially ones that do not meet the TOML standards.

[riley-harper]

See also #35.