test on both pymc3 master and release

[OriolAbril]

Description

We missed a bug that could have been caught testing on latest pymc3 release, given that we do have 2 external CI jobs, I have configured it so that one runs on master and another on latest release.

Checklist

• Follows official PR format
• Changes are listed in changelog

[ahartikainen]

Lgtm

[codecov]

Codecov Report

Merging #1207 into master will increase coverage by 2.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1207      +/-   ##
==========================================
+ Coverage   90.87%   93.10%   +2.22%     
==========================================
  Files          94       94              
  Lines        9480     9480              
==========================================
+ Hits         8615     8826     +211     
+ Misses        865      654     -211     

Impacted Files	Coverage Δ
arviz/data/io_pymc3.py	91.60% <0.00%> (-0.41%)	⬇️
arviz/data/converters.py	68.57% <0.00%> (+1.42%)	⬆️
arviz/data/io_emcee.py	98.33% <0.00%> (+5.00%)	⬆️
arviz/data/io_pystan.py	98.67% <0.00%> (+45.25%)	⬆️

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