STY: Flake8 3.6+ cleanup

[effigies]

Fixes #687.

[coveralls]

Coverage remained the same at 91.851% when pulling 0c4256d on effigies:fix/flake8 into 118d305 on nipy:master.

[codecov-io]

Codecov Report

Merging #695 into master will not change coverage.
The diff coverage is 90%.

@@           Coverage Diff           @@
##           master     #695   +/-   ##
=======================================
  Coverage   88.88%   88.88%           
=======================================
  Files          93       93           
  Lines       11449    11449           
  Branches     1892     1892           
=======================================
  Hits        10176    10176           
  Misses        933      933           
  Partials      340      340

Impacted Files	Coverage Δ
nibabel/nicom/dwiparams.py	71.69% <100%> (ø)	⬆️
nibabel/spatialimages.py	96.33% <100%> (ø)	⬆️
nibabel/xmlutils.py	85.36% <100%> (ø)	⬆️
nibabel/py3k.py	35.71% <100%> (ø)	⬆️
nibabel/quaternions.py	99.02% <100%> (ø)	⬆️
nibabel/affines.py	100% <100%> (ø)	⬆️
nibabel/nifti1.py	91.22% <100%> (ø)	⬆️
nibabel/casting.py	84.74% <50%> (ø)	⬆️

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[effigies]

Minor cleanups, so I'll merge tomorrow unless someone wants to review sooner.