Update myst-parser[linkify] requirement from <3,>=0.18 to >=0.18,<4

Updates the requirements on [myst-parser[linkify]](https://github.com/executablebooks/MyST-Parser) to permit the latest version.
- [Release notes](https://github.com/executablebooks/MyST-Parser/releases)
- [Changelog](https://github.com/executablebooks/MyST-Parser/blob/master/CHANGELOG.md)
- [Commits](executablebooks/MyST-Parser@v0.18.0...v3.0.1)

---
updated-dependencies:
- dependency-name: myst-parser[linkify]
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -119,7 +119,7 @@ develop = [
 ]
 docs = [
   "furo",
-  "myst-parser[linkify]<3,>=0.18",
+  "myst-parser[linkify]>=0.18,<4",
   "sphinx-autobuild==2021.3.14", # Newer versions stopped "watching" appropriately?
   "sphinx-copybutton",
   "sphinx-design-elements<1",