(feat)ci: pypy3.11 support

[kclowes]

🗒️ Description

Allows this repo to run using pypy3.11. The new CI job runs in ~15m, which is generally ~30s-1m slower than our next slowest job. I'm happy to spend more time trying to get it down, but didn't know how much priority to give it here.

🔗 Related Issues or PRs

#1839

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx --with=tox-uv tox -e lint,typecheck,spellcheck,markdownlint

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).

[kclowes]

This is a dependency of pyspelling, and needs to be >=6.0 for pypy. pyspelling doesn't have any restriction on the version, so need to specify here. Could get more specific if we wanted - something along the lines of "lxml>=6.0.0,<7; implementation_name == 'pypy'",

[danceratopz]

Amazing! Really nice to have this!

As a sanity check could you please:

1. Compare cpython and pypy 3.11 fill run times on this branch and cpython on main?
2. Diff the generated fixtures with pypy on this branch to fixtures generated on main? We use the hasher utitlity to diff? This should be adequate:

 uv run fill -m "not slow and not benchmark" -n auto

I think the docs are a bit borked on this, but this comment explains how to do this:
#792 (review)

I absolutely recommend doing this on the hive server for speed!