add scipy-stubs as development dependency

[jorenham]

Description

I noticed that you are using type annotations (which is great), and scipy-stubs can help make that a bit easier less difficult :)

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

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📚 Documentation preview 📚: https://arviz--2445.org.readthedocs.build/en/2445/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.75%. Comparing base (ef7042b) to head (bb5c99d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2445   +/-   ##
=======================================
  Coverage   86.75%   86.75%           
=======================================
  Files         124      124           
  Lines       12941    12941           
=======================================
  Hits        11227    11227           
  Misses       1714     1714           

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[jorenham]

I'm not sure what to make of these CI failures 🤔