Use absolute imports

[MarcoGorelli]

This was discussed but not implemented, and it was pretty easy to write a script to do this so I figured I'd submit a PR

---

I have no strong opinion on whether relative or absolute imports are better (though PEP8 suggests absolute), but I do think that in a large codebase such as PyMC3's then there is a benefit to consistency

[codecov]

Codecov Report

Merging #4292 (d6d7d39) into master (218dd4a) will decrease coverage by 0.00%.
The diff coverage is 98.50%.

@@            Coverage Diff             @@
##           master    #4292      +/-   ##
==========================================
- Coverage   87.57%   87.56%   -0.01%     
==========================================
  Files          88       88              
  Lines       14274    14270       -4     
==========================================
- Hits        12500    12496       -4     
  Misses       1774     1774              

Impacted Files	Coverage Δ
pymc3/step_methods/sgmcmc.py	0.00% <0.00%> (ø)
pymc3/backends/__init__.py	100.00% <100.00%> (ø)
pymc3/backends/base.py	86.92% <100.00%> (ø)
pymc3/backends/report.py	90.90% <100.00%> (ø)
pymc3/backends/tracetab.py	100.00% <100.00%> (ø)
pymc3/blocking.py	69.23% <100.00%> (ø)
pymc3/distributions/__init__.py	100.00% <100.00%> (ø)
pymc3/distributions/bart.py	80.80% <100.00%> (ø)
pymc3/distributions/continuous.py	93.31% <100.00%> (ø)
pymc3/distributions/discrete.py	96.78% <100.00%> (ø)
... and 49 more