Use mypy and ruff from pixi environment

[bouweandela]

Description

Use mypy and ruff from the pixi environment instead of installing them with pre-commit.

Pros

• More accurate mypy checks because all dependencies are installed
• Manage only one environment (pixi) instead of two (pixi and pre-commit)
• No unexpected test breaking because pre-commit does not use lock files for its environments as in Tests failing - github actions pre-commit, mypy - error: "TypeAliasType" not callable #3117
• mypy and ruff can be used without pre-commit. Useful if you want to run these tools with additional options, e.g. ruff check --fix --unsafe-fixes

Cons

• Does not work with pre-commit.ci

We could decide to move away from pre-commit.ci and instead just run the pre-commit hooks on CircleCI as part of the test suite. That would also have the advantage that it has one less CI service, so easier to understand for new contributors.

Link to documentation: https://esmvaltool--3124.org.readthedocs.build/projects/ESMValCore/en/3124/contributing.html#code-quality

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Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.27%. Comparing base (9957f44) to head (919689f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3124   +/-   ##
=======================================
  Coverage   96.26%   96.27%           
=======================================
  Files         280      280           
  Lines       16258    16258           
=======================================
+ Hits        15651    15652    +1     
+ Misses        607      606    -1     

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[valeriupredoi]

thanks bud - a very welcome approach indeed! Approving preemptively (don't pull a Klaus on me since I approve drafts 😆 ) since I'll be away most next week