Switch from toml to tomli/tomllib

[mwgamble]

The toml package is unmaintained, so it was appropriate to move away from it as a dependency. The tomli package is the most appropriate alternative, because it was incorporated into the Python standard library as of Python 3.11, under the name tomllib.

Due to the prevalence of tomli as a compulsory dependency of many other Python code quality tools (such as black, mypy, pytest and pylint), there's a big chance that users will already have it installed if they're using Import Linter. Therefore, tomli is now a compulsory dependency (if you're using versions of Python earlier than 3.11).

This change has several benefits:

• Reduces the overall number of dependencies in the average Python application
• Removes a dependency on an unmaintained project
• Reduces the complexity of configuration-related testing
• Greatly simplifies the tox configuration

Fixes #146

[mwgamble]

@seddonym I've updated the PR as requested :+1:

[mwgamble]

@seddonym Any chance this PR could be merged? That way my grimp types update is based off this work.

[seddonym]

Done, apologies for the delay. Thanks again!

[seddonym]

Hmm, interestingly the merge has caused master mypy checks to fail, because it's now installing the latest version of Grimp. I wonder if we should pin the dependencies on Grimp in the toxfile?

Anyway it sounds like your types update will address these issues, so I'll leave it to you if that's ok?

[mwgamble]

Yep, it's gonna be easiest to just wait a bit for my next PR :+1:

[mwgamble]

Speaking of: https://github.com/seddonym/import-linter/pull/153

By the way I think these failures could have been avoided if the intermediary protocol didn't exist (a la #148)