[FIX] GraphPluginBase error in Python

[mfalkiewicz]

Fixes #2353

Python 3 creates generators instead of lists, which breaks the GraphPluginBase class. It does not allow to submit jobs with SLURMGraph plugin.

[oesteban]

@mfalkiewicz would you mind adding an entry to the CHANGES file? Also, if you commit with [skip ci] in the first line of the commit message, we can avoid re-running all tests and merge directly.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@80d3f05). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #2359   +/-   ##
=========================================
  Coverage          ?   52.19%           
=========================================
  Files             ?      122           
  Lines             ?    24393           
  Branches          ?        0           
=========================================
  Hits              ?    12731           
  Misses            ?    11662           
  Partials          ?        0

Flag	Coverage Δ
#smoketests	52.19% <0%> (?)

Impacted Files	Coverage Δ
nipype/pipeline/plugins/base.py	38.92% <0%> (ø)

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[effigies]

We can update CHANGES before releasing. Happy to merge on tests passing.