Remove theanof.set_theano_conf and instead use the config context

[michaelosthege]

A pm.Model can take a theano_config kwarg and defaults it to {"compute_test_value": "raise"}.
The Theano config was changed using theanof.set_theano_config, which accessed private Theano APIs that were recently refactored on Theano-PyMC.

This PR removes that theanof.set_theano_config entirely and instead assigns the change_flags context as an attribute. The __enter__ and __exit__ methods are called accordingly.

The interesting thing to see in the CI will be if this has any side-effects.

• what are the (breaking) changes: Removal of function, mentioned in RELEASE-NOTES
• important background, or details about the implementation
• test/docstring coverage: Code was removed. It's quite at the core of pm.Model, so if something is wrong the CI will go 🎆

[codecov]

Codecov Report

Merging #4329 (b365b6b) into master (6f15cbb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4329      +/-   ##
==========================================
+ Coverage   87.66%   87.67%   +0.01%     
==========================================
  Files          88       88              
  Lines       14264    14245      -19     
==========================================
- Hits        12504    12489      -15     
+ Misses       1760     1756       -4     

Impacted Files	Coverage Δ
pymc3/model.py	88.90% <100.00%> (+0.02%)	⬆️
pymc3/theanof.py	90.47% <100.00%> (+0.86%)	⬆️

[brandonwillard]

Looks good. My only recommendation is to squash the two commits.