FIX: Constrain environment dictionary to bytes in Windows

[effigies]

Fixes #2068.

Changes proposed in this pull request

• Constrain environment dictionary to bytes in Windows

This filter is run just before the Popen call, which has the advantage that it will be very difficult for a unicode entry to slip through, but the disadvantage that any work that it does will be done many times. Still, it should be a cheap operation.

[codecov-io]

Codecov Report

Merging #2085 into master will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #2085      +/-   ##
==========================================
- Coverage   72.13%   72.13%   -0.01%     
==========================================
  Files        1138     1138              
  Lines       57268    57280      +12     
  Branches     8206     8210       +4     
==========================================
+ Hits        41313    41319       +6     
- Misses      14663    14669       +6     
  Partials     1292     1292

Flag	Coverage Δ
#smoketests	72.13% <33.33%> (-0.01%)	⬇️
#unittests	69.74% <25%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/base.py	83.43% <33.33%> (-0.37%)	⬇️

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