MNT: Drop Python 3.4 support

[effigies]

Summary

This PR does not attempt to hunt down Python 3.4-isms and purge them. Just removes the test battery and updates the package metadata.

Acknowledgment

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

[codecov]

Codecov Report

Merging #3062 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3062      +/-   ##
==========================================
- Coverage   67.72%   67.67%   -0.05%     
==========================================
  Files         344      344              
  Lines       44087    44087              
  Branches     5554     5554              
==========================================
- Hits        29856    29836      -20     
- Misses      13465    13485      +20     
  Partials      766      766

Flag	Coverage Δ
#smoketests	50.29% <100%> (ø)	⬆️
#unittests	65.13% <100%> (-0.05%)	⬇️

Impacted Files	Coverage Δ
nipype/info.py	94.36% <100%> (ø)	⬆️
nipype/interfaces/dcm2nii.py	59.06% <0%> (-6.05%)	⬇️
nipype/utils/filemanip.py	78.97% <0%> (-1.38%)	⬇️

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