ci: Add Flake8 linting to notebooks

[EdCaunt]

@ggorman pointed out that you can now Flake8 lint Jupyter Notebooks. We should probably do this.

[mloubout]

I personally wouldn't. Flake8 is really not adapted to notebooks, where readable and potentially longer lines are preferred.

[EdCaunt]

We could always set a custom rule to ignore "line too long"

[EdCaunt]

Looking at it, there are only about 5 "Line too long" warnings in the whole output, and most are only 5 or so characters over.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.26%. Comparing base (2f18ab8) to head (be6715a).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2484      +/-   ##
==========================================
- Coverage   87.26%   87.26%   -0.01%     
==========================================
  Files         238      238              
  Lines       45278    45271       -7     
  Branches     4022     4022              
==========================================
- Hits        39512    39505       -7     
  Misses       5085     5085              
  Partials      681      681              

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[review-notebook-app]

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[mloubout]

This diff is way too big for just a few line change. Also those depa should be in requirements-testing.txt not hardcoded in the yaml