fix pair_plot for mixed discrete and continuous variables

[aloctavodia]

When plotting mixed variables plot_pair produced a wrong result, basically the type of the variable was taken from the first variable and applied to the rest.

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1434 into master will increase coverage by 0.02%.
The diff coverage is 93.02%.

@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
+ Coverage   91.97%   91.99%   +0.02%     
==========================================
  Files         105      105              
  Lines       11173    11184      +11     
==========================================
+ Hits        10276    10289      +13     
+ Misses        897      895       -2     

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/pairplot.py	92.10% <90.90%> (+0.08%)	⬆️
arviz/plots/backends/bokeh/pairplot.py	88.48% <92.00%> (+1.66%)	⬆️
arviz/plots/pairplot.py	88.00% <100.00%> (ø)

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