Improve sampling coverage

[MarcoGorelli]

Added test cases for pm.sample(idata_kwargs=....) and for random_seed=-1.

While doing this, I also came across some outdated Python2 constructs (which I changed)

[MarcoGorelli]

unnecessary pass statement

[michaelosthege]

I like to put them there to mark the end of a test/function (just like an unnecessary return).

In my opinion they have a few advantages:

• trailing comments are collapsed together with the code
• because the pass in a test, or return in a function was put there on purpose, you don't wonder if somebody just stopped writing, or accidentally deleted some code

Not saying you must put it back in - just showing a maybe new perspective :)

[twiecki]

To me that looks weird and confusing.

[codecov]

Codecov Report

Merging #4270 (fa42e7e) into master (edbafaa) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4270      +/-   ##
==========================================
+ Coverage   87.44%   87.51%   +0.06%     
==========================================
  Files          88       88              
  Lines       14342    14333       -9     
==========================================
+ Hits        12542    12543       +1     
+ Misses       1800     1790      -10     

Impacted Files	Coverage Δ
pymc3/sampling.py	86.21% <100.00%> (+0.70%)	⬆️
pymc3/parallel_sampling.py	86.79% <0.00%> (+0.94%)	⬆️

[michaelosthege]

I like to put them there to mark the end of a test/function (just like an unnecessary return).

In my opinion they have a few advantages:

• trailing comments are collapsed together with the code
• because the pass in a test, or return in a function was put there on purpose, you don't wonder if somebody just stopped writing, or accidentally deleted some code

Not saying you must put it back in - just showing a maybe new perspective :)

[michaelosthege]

Looks generally good! Just a few questions/details in the comments and then we'll be ✔️

[twiecki]

Thanks @MarcoGorelli!