RF: Drop various remaining compatibilities for Python < 3.5

[effigies]

Summary

In merging master into dev/2.0, I saw a Python >= 3.2 check, indicating a less-than-thorough purge of all obsolete version checks. Here I grepped for sys.version and removed any checks that no longer apply for Python < 3.5. (I think < 3.4.1 is the actual highest check, so we're not in the range of distinguishing 3.5 and 3.6.)

The main thing to-do is to change to_str() to str() everywhere, and remove that function fully, but I want to get tests running to make sure I'm not breaking anything.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov-io]

Codecov Report

Merging #2831 into dev/2.0 will increase coverage by 0.44%.
The diff coverage is 86.66%.

@@             Coverage Diff             @@
##           dev/2.0    #2831      +/-   ##
===========================================
+ Coverage    66.67%   67.11%   +0.44%     
===========================================
  Files          338      338              
  Lines        42879    42798      -81     
  Branches      5363     5338      -25     
===========================================
+ Hits         28588    28726     +138     
+ Misses       13566    13378     -188     
+ Partials       725      694      -31

Flag	Coverage Δ
#smoketests	49.95% <75.86%> (+1.17%)	⬆️
#unittests	64.5% <86.66%> (+0.08%)	⬆️

Impacted Files	Coverage Δ
nipype/info.py	93.75% <ø> (+1.21%)	⬆️
nipype/sphinxext/plot_workflow.py	13.79% <0%> (+0.11%)	⬆️
nipype/pipeline/plugins/tools.py	84.5% <100%> (ø)	⬆️
nipype/utils/profiler.py	18.49% <100%> (+0.47%)	⬆️
nipype/interfaces/base/specs.py	93.01% <100%> (ø)	⬆️
nipype/pipeline/engine/utils.py	81.93% <100%> (+3.45%)	⬆️
nipype/utils/filemanip.py	78.79% <100%> (+8.5%)	⬆️
nipype/interfaces/base/support.py	79.41% <100%> (ø)	⬆️
nipype/pipeline/engine/nodes.py	84.42% <75%> (ø)	⬆️
nipype/pipeline/engine/workflows.py	78.81% <81.81%> (+0.33%)	⬆️
... and 10 more

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