Use contourpy for Bokeh contour plots

[ahartikainen]

Description

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--2104.org.readthedocs.build/en/2104/

[codecov]

Codecov Report

Merging #2104 (2eb46f1) into main (5ca7643) will decrease coverage by 0.02%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main    #2104      +/-   ##
==========================================
- Coverage   90.79%   90.76%   -0.03%     
==========================================
  Files         117      117              
  Lines       12488    12497       +9     
==========================================
+ Hits        11338    11343       +5     
- Misses       1150     1154       +4     

Impacted Files	Coverage Δ
arviz/rcparams.py	92.56% <60.00%> (-0.72%)	⬇️
arviz/plots/backends/bokeh/kdeplot.py	94.70% <75.00%> (-1.19%)	⬇️
arviz/stats/stats.py	95.87% <100.00%> (ø)

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[zaxtax]

Are there any blockers on this issue?