MNT: fix flake8 warning

[DimitriPapadopoulos]

Except E501 (lines too long).

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1f4683a) 31.18% compared to head (a0e766a) 31.18%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #349   +/-   ##
=======================================
  Coverage   31.18%   31.18%           
=======================================
  Files          20       20           
  Lines        1456     1456           
  Branches      206      206           
=======================================
  Hits          454      454           
  Misses        987      987           
  Partials       15       15           

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