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?