Adding jupytext to pre commit to apply black to myst notebook code

[davidorme]

Description

This PR:

• Adds jupytext --pipe black as a pre-commit step for the markdown files in docs/source, which auto-formats any Python in {code-cell} directives.
• Updates markdown files to set the notebook language where needed and add Python3 to code cells.
• Updates the metadata retained in markdown files (pyproject.toml: [tool.jupytext])

This is a little finicky and the implementation is being looked at:
mwouts/jupytext#1267

It is possible that we might be able to use ruff here but at present only the .ipynb format for notebooks is supported by ruff-format.

astral-sh/ruff#8800 (comment)

Fixes #154

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.18%. Comparing base (1b31149) to head (8729fab).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #607   +/-   ##
========================================
  Coverage    95.18%   95.18%           
========================================
  Files           74       74           
  Lines         4380     4380           
========================================
  Hits          4169     4169           
  Misses         211      211           

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[jacobcook1995]

Skimmed the file changes at it looks sensible to me, happy for this to be merged once the docs build failures are fixed