remove pin from mpl 3.3

[ahartikainen]

I forgot to do this in previous PR.

[codecov]

Codecov Report

Merging #1352 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1352      +/-   ##
==========================================
- Coverage   91.80%   91.80%   -0.01%     
==========================================
  Files         101      101              
  Lines       10636    10635       -1     
==========================================
- Hits         9764     9763       -1     
  Misses        872      872              

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/pairplot.py	92.02% <100.00%> (-0.05%)	⬇️
arviz/plots/backends/matplotlib/ppcplot.py	98.07% <100.00%> (ø)

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