fix(pyup): Update myst-parser to 0.18.0

[pyup-bot]

This PR updates myst-parser from 0.17.2 to 0.18.0.

Changelog

0.18.0

Full Changelog: [v0.17.2...v0.18.0](https://github.com/executablebooks/MyST-Parser/compare/v0.17.2...v0.18.0)

This release adds support for Sphinx v5 (dropping v3), restructures the code base into modules, and also restructures the documentation, to make it easier for developers/users to follow.

It also introduces **document-level configuration**  *via* the Markdown top-matter, under the `myst` key.
See the [Local configuration](docs/configuration.md) section for more information.

Breaking changes

This should not be breaking, for general users of the sphinx extension (with `sphinx>3`),
but will be for anyone directly using the Python API, mainly just requiring changes in import module paths.

The `to_docutils`, `to_html`, `to_tokens` (from `myst_parser/main.py`) and `mock_sphinx_env`/`parse` (from `myst_parser.sphinx_renderer.py`) functions have been removed, since these were primarily for internal testing.
Instead, for single page builds, users should use the docutils parser API/CLI (see [](docs/docutils.md)),
and for testing, functionality has been moved to <https://github.com/chrisjsewell/sphinx-pytest>.

The top-level `html_meta` and `substitutions` top-matter keys have also been deprecated (i.e. they will still work but will emit a warning), as they now form part of the `myst` config, e.g.

yaml
---
html_meta:
"description lang=en": "metadata description"
substitutions:
key1: I'm a **substitution**
---


is replaced by:

yaml
---
myst:
html_meta:
 "description lang=en": "metadata description"
substitutions:
 key1: I'm a **substitution**
---


Key PRs

- ♻️📚 Restructure code base and documentation (566)
- ⬆️ Drop Sphinx 3 and add Sphinx 5 support (579)
- 🐛 FIX: `parse_directive_text` when body followed by options (580)
- 🐛 FIX: floor table column widths to integers (568), thanks to Jean-Abou-Samra!

Links

• PyPI: https://pypi.org/project/myst-parser
• Changelog: https://pyup.io/changelogs/myst-parser/