Pin nbformat to 5.0.8 to avoid regression.

[lesteve]

nbformat 5.1 adds random ids to cells which creates issues with jupyter-cache.

This manifests itself as cell outputs not being shown (e.g. df.head() output is blank below):

Warnings like:

WARNING: Notebook code has no file extension metadata, defaulting to `.txt`
WARNING: Couldn't find cache key for notebook file python_scripts/02_numerical_pipeline_ex_01.py. Outputs will not be inserted.

Thanks @Phaide for noticing!

This problem is mentioned here for example: mwouts/jupytext#715

[lesteve]

Outputs are shown: https://762-246063957-gh.circle-artifacts.com/0/jupyter-book/python_scripts/02_numerical_pipeline_introduction.html

Merging!