JNB+DEV: Updates to nbqa for notebook pre-commit

[scottclowe]

• Specify that notebooks should be blackened targeting python 2.7, and >=3.5.
• Sanity check notebooks with flake8.
• Fix some flake8 errors in the notebooks.

[codecov]

Codecov Report

Merging #261 (e4745bc) into master (f920357) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #261   +/-   ##
=======================================
  Coverage   94.12%   94.12%           
=======================================
  Files           8        8           
  Lines        1209     1209           
  Branches      302      302           
=======================================
  Hits         1138     1138           
  Misses         36       36           
  Partials       35       35           

Flag	Coverage Δ
nbsmoke	59.88% <ø> (ø)
unittests	93.79% <ø> (ø)

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