Bump docs constraints pip-tools runtime to 3.13

[webknjaz]

This is needed because Dependabot and other GH Code Security tooling reads the file header to determine the Python version to use.

Currently, it's unable to bump pip in the docs env [1] supposedly due to this.

Contributor checklist

• Included tests for the changes.
• A change note is created in changelog.d/ (see changelog.d/README.md for instructions) or the PR text says "no changelog needed".

Maintainer checklist

• If no changelog is needed, apply the bot:chronographer:skip label.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[Copilot]

Pull Request Overview

This PR updates the autogenerated comment in the documentation requirements file to reflect that it was compiled using Python 3.13 instead of Python 3.11. This is purely a metadata update in a comment, with no functional code changes.

• Updated pip-compile comment to indicate Python 3.13 was used for requirements generation

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[sirosen]

Can we also set base_python for the tox env which updates this file?

[webknjaz]

Can we also set base_python for the tox env which updates this file?

Not sure. base_python has proven difficult to maintain in the past. Plus it might not be available for contributors while Sphinx would be perfectly functional on other versions otherwise. I think that having the version set for the job in GHA already achieves what we want.

If anything, we could work on a warning emitted from a toxfile.py on failure with a hint if the current Python version differs from what's in GHA/RTD configs.