A principe change in this release is to updates the requirements of myst-parser from `sphinx>=2,<4` to `sphinx>=3,<5`.

Instead of removing all `$` processing for the whole project,

during MyST document parsing, the top-level section is now given the classes `tex2jax_ignore` and `mathjax_ignore` (turning off default MathJax processing of all HTML elements)

and MathJax is then configured to process elements with the `tex2jax_process|mathjax_process|math` classes.

See [the math syntax guide](docs/using/syntax.md#math-shortcuts) for further information.

The `myst_url_schemes` default is now: `("http", "https", "mailto", "ftp")`.

This means that only these URL will be considered as external (e.g. `[](https://example.com)`),

and references like `[](prefix:main)` will be considered as internal references.

Set `myst_url_schemes = None`, to revert to the previous default.

Use this option to specify a custom function to auto-generate heading anchors (see [Auto-generated header anchors](docs/using/syntax-optional.md#auto-generated-header-anchors)).

Thanks to [@jpmckinney](https://github.com/jpmckinney)!