[FIX] Python 3 compatibility of gantt chart generator

[oesteban]

A minor revision of this utility.

[codecov-io]

Codecov Report

Merging #1907 into master will increase coverage by 0.36%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #1907      +/-   ##
==========================================
+ Coverage   72.55%   72.92%   +0.36%     
==========================================
  Files        1064     1061       -3     
  Lines       54164    53773     -391     
  Branches     7809     7737      -72     
==========================================
- Hits        39301    39216      -85     
+ Misses      13645    13343     -302     
+ Partials     1218     1214       -4

Flag	Coverage Δ
#smoketests	72.92% <22.22%> (+0.36%)	⬆️
#unittests	70.38% <22.22%> (+0.35%)	⬆️

Impacted Files	Coverage Δ
nipype/utils/draw_gantt_chart.py	27.02% <22.22%> (+0.5%)	⬆️
nipype/interfaces/afni/utils.py	74.77% <0%> (-0.85%)	⬇️
nipype/pipeline/engine/workflows.py	76.41% <0%> (-0.09%)	⬇️
...ype/interfaces/tests/test_auto_FreeSurferSource.py	100% <0%> (ø)	⬆️
.../interfaces/freesurfer/tests/test_auto_ReconAll.py	100% <0%> (ø)	⬆️
nipype/algorithms/modelgen.py	73.36% <0%> (ø)	⬆️
nipype/sphinxext/plot_workflow.py
...nterfaces/freesurfer/tests/test_auto_MRIsExpand.py
nipype/sphinxext/__init__.py
nipype/interfaces/io.py	50.93% <0%> (ø)	⬆️
... and 3 more

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