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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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[bot]]

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[EdCaunt]

This diff is way too big for just a few line change.

Yeah, I messed up adding the changes in one of the notebooks somehow. Will fix in due course

[EdCaunt]

Also those depa should be in requirements-testing.txt not hardcoded in the yaml

I was just going off how the Flake8 workflow already works, can change this