fix bug in io_pymc3 and add warning for PyMC3<3.9

[OriolAbril]

Description

bugfix for io_pymc3, there was one self.nchains that should have been self.ndraws. Fixes #1202

Checklist

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

[codecov]

Codecov Report

Merging #1203 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1203      +/-   ##
==========================================
- Coverage   93.12%   93.10%   -0.02%     
==========================================
  Files          94       94              
  Lines        9478     9480       +2     
==========================================
  Hits         8826     8826              
- Misses        652      654       +2     

Impacted Files	Coverage Δ
arviz/data/io_pymc3.py	92.00% <0.00%> (-0.75%)	⬇️

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