🔧 Modernize packaging, tooling and CI

[chrisjsewell]

All CI jobs are currently failing: Python 3.7–3.9 are no longer available on ubuntu-latest runners (the jobs die at "Set up Python"), and the one job that does run (py3.10) fails in pytest against modern Sphinx/docutils. This PR brings the project's tooling in line with MyST-Parser's current setup and makes CI green again.

Packaging

• Require Python >=3.11 and sphinx>=8,<10; update classifiers accordingly
• Fix the Homepage URL (repo moved to sphinx-extensions2) and add a Documentation URL
• flit_core >=3.4 and sdist excludes for docs//tests/
• Drop the docutils <0.18 findall/traverse compatibility shim (_compat.py) — no longer reachable with the new support floor

Lint / format

• Replace black + isort + flake8 + pyupgrade with ruff-check + ruff-format (same rule selection as MyST-Parser); supersedes [pre-commit.ci] pre-commit autoupdate #15
• Update pre-commit-hooks to v6

CI

• New pre-commit job
• Test matrix: py3.11–3.14 × sphinx >=8,<9 / >=9,<10, plus two Windows jobs
• actions/checkout@v6 / setup-python@v6 (were v2/v1)
• New PyPI publish job on v* tags using trusted publishing — ⚠️ requires adding a trusted publisher for this repo/workflow (environment pypi) in the PyPI project settings before the next release
• Dropped the Codecov upload for now (needs a CODECOV_TOKEN with current codecov-action versions); coverage still reports in the job log

Tests (the py3.10 pytest failures)

• The LaTeX build tests hardcoded python.tex, but Sphinx ≥7.2 renamed the default project (projectnamenotset.tex) — now globs for the .tex file
• Doctree fixtures asserted is_subfigure="True", but docutils 0.22 serializes bool attributes as "1" (0.21 as "True"), so no single fixture could pass the whole matrix — the attribute is now the string "true", which serializes identically on all versions
• ✅ All 15 tests pass locally on both sphinx 8.2.3 / docutils 0.21.2 and sphinx 9.0.4 / docutils 0.22.4

tox / Read the Docs

• tox: py{311,312,313,314}-sphinx{8,9} and docs-{update,clean} environments
• RTD: ubuntu-24.04 / Python 3.12, and the now-mandatory sphinx.configuration key (docs build verified warning-free locally with -nW on Sphinx 9 + latest myst-parser/furo)

Also updates AGENTS.md to match (removed _compat.py, ruff, new tox env names).

Deliberately not included, to leave the contributor PRs intact: the debug print() removal (#19) and the parallel read/write declaration (#16) — both still apply cleanly on top of this.