myst_parser is not needed to run tests.

--- sybil-10.1.0/pyproject.toml.orig
+++ sybil-10.1.0/pyproject.toml
@@ -26,7 +26,6 @@
 [dependency-groups]
 dev = [
     "mypy>=1.0",
-    "myst-parser",
     "pytest>=8",
     "pytest-cov>=6",
     "ruff>=0.15.7",