fix minor typing issue in stats.stats.compare

[jorenham]

Description

I noticed in scipy/scipy-stubs#881 (comment) that there's a minor mypy issue in arviz.stats.stats.compare where the weights name is used for multiple types. That's affects how mypy infers the weights type as, which is what eventually led to these two reported mypy errors. As you can see, the error messages are rather confusing, and are reported at a different location. So I figured I'd spare you the debugging effort, and might as well fix it myself 🤷🏻

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

---

📚 Documentation preview 📚: https://arviz--2477.org.readthedocs.build/en/2477/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.82%. Comparing base (01c8b94) to head (bd6cefb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2477   +/-   ##
=======================================
  Coverage   86.82%   86.82%           
=======================================
  Files         124      124           
  Lines       13014    13014           
=======================================
  Hits        11300    11300           
  Misses       1714     1714           

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[aloctavodia]

thanks @jorenham