Sphinx 7.4.0 breaks parsing of MyST-Parser (and probably others) configuration in conf.py

[jfbu]

Describe the bug

Bad output:

Good output:

How to Reproduce

conf.py:

extensions = ['myst_parser',
]

myst_enable_extensions = ["colon_fence"]
source_suffix = ['.rst', '.md']

index.md:

# La FAQ LaTeX francophone !


:::{only} html
Hello
:::

## Section

:::{admonition} Here's my title
:class: tip

Here's my admonition content.
:::

Environment Information

Git bisect blames 82edc3d

Running Sphinx v7.4.0+/82edc3d38
Sphinx version: 7.4.0+/82edc3d38
Python version: 3.12.3 (CPython)
Docutils version: 0.20.1 
Pygments version: 2.18.0
Jinja2 version: 3.1.4
MyST-parser version: 3.0.1

Sphinx extensions

extensions = ['myst_parser',
]

myst_enable_extensions = ["colon_fence"]
source_suffix = ['.rst', '.md']

[jfbu]

I am sorry I did not catch that, but right prior to 7.4.0 release I was on my PR #12508 branch all of the time and did use it on big project regularly until a few days ago and clearly the branch was without the bad commit. Perhaps at very end it did after I had done one or two merges of master (I have not checked that, speaking hypothetically) but at this stage (last two days circa) I was only compiling via latexpdf some small testing project for finalizing admonition styles and was not using Markdown/MyST-Parser for input.

[AA-Turner]

Your source_suffix should ideally be:

source_suffix = {
    '.rst': 'restructuredtext',
    '.md': 'markdown',
}

However the list-style has not yet been fully deprecated and removed, so I will restore support.

[jfbu]

@AA-Turner Indeed! I fetched that config from the big project where the problem was first detected! Issue is much less serious I first thought then...

[AA-Turner]

Sphinx 7.4.3 has been released with a fix.

A