release of 0.15.0 was blocked by myst

[12rambau]

myst is triggering a new bug in our CI when used with 3.12. This bug is non existent with other Python Versions

https://github.com/pydata/pydata-sphinx-theme/actions/runs/7408284101

Unexpected warning: WARNING: The config value `myst_enable_extensions' has type `list'; expected `Any'.

It confirms I need to finish #1530 but in the meantime lets pin myst version to a working number so we can release v0.15.1

[drammock]

This is not a myst-parser problem but rather a sphinx problem. See similar issues that have come up before: sphinx-doc/sphinx#9188

[drammock]

(notice that myst-parser is version 2.0.0 in both the passing python 3.9 build and the failing python 3.12 build). The difference is Sphinx 6.1.0 vs 7.3.0

[12rambau]

yep I saw it afterward when I looked into the WARNING in detail