Pin the Python version

[tsmbland]

Just had a go setting everything up on a new laptop, following the dev instruction. All super clear and everything worked really well with one exception, when I got to running pre-commit hooks with just pre-commit:

ERROR: Package 'check-jsonschema' requires a different Python: 3.9.6 not in '>=3.10'

I was able to fix it by running

uv tool install --python 3.12 pre-commit --force

but perhaps better to pin the Python version using a .python-version file, as described here. Unless there's some reason why we shouldn't pin the Python version?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.32%. Comparing base (ceb32bc) to head (21a6876).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1195   +/-   ##
=======================================
  Coverage   87.32%   87.32%           
=======================================
  Files          57       57           
  Lines        7726     7726           
  Branches     7726     7726           
=======================================
  Hits         6747     6747           
  Misses        673      673           
  Partials      306      306           

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[Copilot]

Pull request overview

Adds a repository-level Python version pin to make uv/pre-commit setup reproducible across developer machines and avoid failures from using an older system Python.

Changes:

• Introduces a .python-version file specifying Python 3.12 for tooling.

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[alexdewar]

but perhaps better to pin the Python version using a .python-version file, as described here. Unless there's some reason why we shouldn't pin the Python version?

No, I think this is a good idea!

In fact, maybe we should consider having a pyproject.toml file and pinning the versions of Python dependencies there, as well as the Python version. (I've just been adding dependencies to scripts directly like this so far.) I can imagine that a few of the docs-related scripts might just stop working in a few years, which will mean we have the annoyance of backporting patches in order to build old docs. What do you think?

Either way, this is an improvement, so I think we should merge and worry about that later.