Apply Repo-Review rule PP006

The dev dependency group should be defined

Author: DimitriPapadopoulos

pyproject.toml

@@ -52,7 +52,7 @@ Paper = "https://doi.org/10.1038/s41592-018-0235-4"
 "Docker Images" = "https://hub.docker.com/r/nipreps/fmriprep/tags/"
 NiPreps = "https://www.nipreps.org/"
 
-[project.optional-dependencies]
+[dependency-groups]
 doc = [
     "pydot >= 1.2.3",
     "sphinx >= 5",