[mpxpy] Automate PyPI releases; add convert_to_xlsx and include_hyperlinks

[mattmathpix]

What

Automate mpxpy's PyPI releases and add two missing API options.

Release automation

New .github/workflows/release.yml, modeled on the mathpix-cli release workflow:

• Version: derived from the git tag via setuptools-scm (pyproject.toml uses dynamic = ["version"]), so there is no version to bump by hand. The tag is the single source of truth.
• Trigger: push a vX.Y.Z tag.
• Guards (fail before anything is published): tag is valid semver, is on master, and is newer than the last release.
• Steps: install → run offline unit tests → python -m build → twine check → publish to PyPI → create a published (non-draft) GitHub release with generated notes.
• Auth: a PYPI_API_TOKEN repo secret (same __token__ credential deploy.sh uses today).

Cutting a release is just:

git checkout master && git pull
git tag -a v0.0.23 -m "mpxpy v0.0.23"
git push origin v0.0.23

New API options (all verified against ocr-api)

• convert_to_mmd_overlay_pdf on pdf_new for the new mmd.overlay.pdf format from the accessible-PDF release: your submitted PDF returned with an invisible, screen-reader-ready text layer (PDF/UA tagged by default; PDF input only). Adds to_mmd_overlay_pdf_file / to_mmd_overlay_pdf_bytes on Pdf and File; the format's options pass through the existing conversion_options["mmd.overlay.pdf"] dict.
• convert_to_xlsx on pdf_new and conversion_new, plus to_xlsx_file / to_xlsx_bytes on Pdf and Conversion. xlsx was the only supported conversion format without a convenience flag.
• include_hyperlinks on pdf_new (extracts PDF link annotations into per-line links data); added to _PDF_REQUEST_OPTION_KEYS.

Changelog updated. The first tag must be v0.0.23 or higher, since 0.0.22 is already on PyPI.

One-time setup required before the first tagged release

Add the existing PyPI token as a repo secret named PYPI_API_TOKEN
(Settings → Secrets and variables → Actions). Until it exists the publish step fails.

Testing

• 87/87 offline unit tests pass (incl. 3 new ones covering the options).
• With a v0.0.23 tag, python -m build stamps the version 0.0.23 into mpxpy-0.0.23.tar.gz/.whl; twine check passes.

Triggered by user request to automate mpxpy PyPI releases via GH Actions and add missing API options.