BF: Update dcmstack interface for Py3 / newer pydicom

[moloney]

Summary

Fixes issues with dcmstack interface on Python3 with newer pydicom.

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

[codecov]

Codecov Report

Base: 63.63% // Head: 63.62% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (071a40e) compared to base (7382b3d).
Patch coverage: 33.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3541      +/-   ##
==========================================
- Coverage   63.63%   63.62%   -0.02%     
==========================================
  Files         309      309              
  Lines       40873    40900      +27     
  Branches     5381     5381              
==========================================
+ Hits        26011    26021      +10     
- Misses      13838    13855      +17     
  Partials     1024     1024              

Impacted Files	Coverage Δ
nipype/interfaces/dcmstack.py	35.10% <33.33%> (ø)
nipype/interfaces/cmtk/cmtk.py	17.75% <0.00%> (-0.15%)	⬇️
nipype/interfaces/cmtk/nx.py	17.73% <0.00%> (-0.07%)	⬇️
nipype/interfaces/cmtk/convert.py	27.01% <0.00%> (-0.05%)	⬇️
nipype/interfaces/cmtk/nbs.py	32.98% <0.00%> (+0.02%)	⬆️
nipype/pipeline/plugins/base.py	58.88% <0.00%> (+0.66%)	⬆️

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[effigies]

Thanks, Brendan. It looks like this doesn't get exercised by any tests. Is it feasible to add a test that would catch a future breakage?