fix: Use non-gui matplotlib backend for tests to avoid hanging on ipywidgets interact

[matthewfeickert]

Description

• To avoid having the notebook tests hang on the ipywidgets interact sections of InterpolationCodes.ipynb set the matplotlib backend to the non-gui Agg which disallows the creation of the gui elements that are causing the problem. This also speeds up runtime as no gui-elements are created for the notebook tests now.
• Remove '%pylab inline' from InterpolationCodes.ipynb as pylab is very deprecated.
• Add --verobse flag to notebook tests to make it easier to track the progress of the notebooks in the CI logs.

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• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

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• Summarize commit messages into a comprehensive review of the PR

* Avoid the notebook tests hanging on the ipywidgets interact sections of `InterpolationCodes.ipynb`
  by setting the matplotlib backend to the non-gui Agg which disallows the creation of the gui elements
  that are causing the problem.
* Remove '%pylab inline' from InterpolationCodes.ipynb as pylab is very deprecated.
* Add --verobse flag to notebook tests to make it easier to track the progress of the notebooks in the
  CI logs.

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[matthewfeickert]

Bug reproducible with

$ jupyter execute docs/examples/notebooks/learn/InterpolationCodes.ipynb
[NbClientApp] Executing docs/examples/notebooks/learn/InterpolationCodes.ipynb
[NbClientApp] Executing notebook with kernel: python3

which then hangs.

[matthewfeickert]

Freezing on the interact call of

from ipywidgets import interact

...

x = interact(
    lambda alpha: plot_interpolated_histogram(alpha, down_hist, nom_hist, up_hist),
    alpha=(-1, 1, 0.1),
)

of https://github.com/scikit-hep/pyhf/blob/3eef1fff2e6d4ccf474bbe51e2211cf690752b82/docs/examples/notebooks/learn/InterpolationCodes.ipynb for ipywidgets v8.0.2.

[matthewfeickert]

Seems to be a general problem with interact https://ipywidgets.readthedocs.io/en/8.0.2/examples/Using%20Interact.html

as

$ curl -sL https://raw.githubusercontent.com/jupyter-widgets/ipywidgets/8.0.2/docs/source/examples/Using%20Interact.ipynb -o /tmp/Using-Interact.ipynb
$ jupyter execute /tmp/Using-Interact.ipynb                                                                                                           
[NbClientApp] Executing /tmp/Using-Interact.ipynb
[NbClientApp] Executing notebook with kernel: python3  # ...hangs

[matthewfeickert]

Temporary workaround locally:

$ jupytext --to py:percent docs/examples/notebooks/learn/InterpolationCodes.ipynb
[jupytext] Reading docs/examples/notebooks/learn/InterpolationCodes.ipynb in format ipynb
[jupytext] Updating the timestamp of docs/examples/notebooks/learn/InterpolationCodes.py
$ MPLBACKEND=agg python docs/examples/notebooks/learn/InterpolationCodes.py
interactive(children=(FloatSlider(value=0.0, description='alpha', max=1.0, min=-1.0), Output()), _dom_classes=('widget-interact',))
[[ 8.   8.2  8.4  8.6  8.8  9.   9.2  9.4  9.6  9.8 10. ]]
[[10.  10.3 10.6 10.9 11.2 11.5 11.8 12.1 12.4 12.7 13. ]]
[[15.  15.5 16.  16.5 17.  17.5 18.  18.5 19.  19.5 20. ]]
/home/feickert/Code/GitHub/pyhf/docs/examples/notebooks/learn/InterpolationCodes.py:221: UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.
  plt.show()
interactive(children=(FloatSlider(value=0.0, description='alpha', max=1.0, min=-1.0), Output()), _dom_classes=('widget-interact',))

also just

$ MPLBACKEND=agg jupyter execute docs/examples/notebooks/learn/InterpolationCodes.ipynb
[NbClientApp] Executing docs/examples/notebooks/learn/InterpolationCodes.ipynb
[NbClientApp] Executing notebook with kernel: python3