[FIX] Update pytest req'd version to 3.6

[oesteban]

Fixes #2826.

Summary

Fixes # .

List of changes proposed in this PR (pull-request)

Acknowledgment

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

[codecov-io]

Codecov Report

Merging #2827 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2827      +/-   ##
==========================================
+ Coverage   67.45%   67.47%   +0.02%     
==========================================
  Files         341      341              
  Lines       43343    43343              
  Branches     5375     5375              
==========================================
+ Hits        29235    29247      +12     
+ Misses      13409    13399      -10     
+ Partials      699      697       -2

Flag	Coverage Δ
#smoketests	50.55% <100%> (ø)	⬆️
#unittests	64.89% <100%> (+0.04%)	⬆️

Impacted Files	Coverage Δ
nipype/info.py	94.11% <100%> (ø)	⬆️
nipype/interfaces/dcm2nii.py	48.13% <0%> (+0.93%)	⬆️
nipype/testing/utils.py	91.37% <0%> (+1.72%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	66% <0%> (+4.5%)	⬆️

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[effigies]

That does seem to fix it... Since we don't depend on pytest-xdist>=1.25, we don't, strictly speaking, need pytest>=3.6, so the question is whether to update the requirement or just update the Travis environment to satisfy the dependency, either by upgrading pytest or downgrading pytest-xdist. I'm not sure it's worth caring about, but pytest 3.6 is only 6 months old. As a matter of fussiness, it's nice not to force people to upgrade a bunch of things in their environment to upgrade nipype unless we really need to.

[oesteban]

They are adding a lot of very cool fixtures that can help make the tests faster to write. I particularly don't mind updating this one.

[effigies]

Okay. Fine with me.