plot_posterior: fix overlap of hdi and rope

[aloctavodia]

Fixes #1262

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

[codecov]

Codecov Report

Merging #1263 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1263   +/-   ##
=======================================
  Coverage   93.21%   93.21%           
=======================================
  Files          98       98           
  Lines        9734     9734           
=======================================
  Hits         9074     9074           
  Misses        660      660           

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/posteriorplot.py	98.09% <100.00%> (ø)

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[AlexAndorra]

LGTM, thanks Osvaldo 👌
Will merge later today in case anybody has some comments