updates for numpy 1.19 compatibility

[OriolAbril]

Description

Test failed in #1255 due to numpy latest version, 1.19, this should fix the issue.

Checklist

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

[codecov]

Codecov Report

Merging #1256 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1256   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files          98       98           
  Lines        9715     9715           
=======================================
  Hits         9046     9046           
  Misses        669      669           

Impacted Files	Coverage Δ
arviz/numeric_utils.py	97.59% <100.00%> (ø)

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