feat: use tomli on Python < 3.15 for TOML 1.1 support

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

TOML 1.1 was recently released.

tomli added support for it in 2.4.0, but for tomllib that is in the standard library, it will only be added in Python 3.15.

There are currently debates about the impact on the packaging ecosystem (https://discuss.python.org/t/adopting-toml-1-1/105624), but on our side, since we only read TOML, and don't write anything, there is no reason not to be able to both parse 1.0 and 1.1 IMO.

So this PR requires tomli on all versions of Python before 3.15. I did not bump tomli lower bound to >=2.4.0 because some packages might explicitly opt out from tomli 2.4.0+, so I want to make sure we don't create dependency resolutions issues in some projects.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.6%. Comparing base (d6ec400) to head (a4d15f7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1446     +/-   ##
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+ Coverage   93.4%   93.6%   +0.1%     
=======================================
  Files         38      38             
  Lines       1036    1034      -2     
  Branches     103     102      -1     
=======================================
  Hits         968     968             
+ Misses        54      53      -1     
+ Partials      14      13      -1     

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