Fixes before releasing

[OriolAbril]

Description

Fixes from_pymc3 compatibility with theano-pymc>=1.1 and allows going back to the original behaviour of from_pymc3 with DensityDist if desired.

Tagging also @michaelosthege for review

Checklist

• Does the PR follow official
  PR format?
• Is the code style correct (follows pylint and black guidelines)?
• Is the fix listed in the Maintenance and fixes
  section of the changelog?

[codecov]

Codecov Report

Merging #1495 (cfdf83f) into master (f7651e0) will decrease coverage by 0.52%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1495      +/-   ##
==========================================
- Coverage   91.96%   91.44%   -0.53%     
==========================================
  Files         105      105              
  Lines       11244    11391     +147     
==========================================
+ Hits        10341    10416      +75     
- Misses        903      975      +72     

Impacted Files	Coverage Δ
arviz/data/io_pymc3.py	92.19% <83.33%> (-0.25%)	⬇️
arviz/rcparams.py	85.45% <0.00%> (-7.68%)	⬇️
arviz/wrappers/base.py	18.36% <0.00%> (-2.79%)	⬇️

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[michaelosthege]

@OriolAbril FYI I can't merge this.

[rpgoldman]

I suggested one minor tweak, but it looks good to me other than that.

[canyon289]

Should there be tests, even if theyre not run in CI

[michaelosthege]

The changes look good. The CI failure looks unrelated to the changes from this PR

[michaelosthege]

@OriolAbril can this be merged? The PR pymc-devs/pymc#4405 is blocked by the ArviZ release..