build: Use 'dev' dependency group for PEP 735 compliance

Only for development and build time dependencies, not run time dependencies
that are meant to be published with the package.

.github/workflows/tests.yaml

@@ -22,10 +22,7 @@ jobs:
         uses: actions/setup-python@v6
         with:
           python-version: "3.x"
-      - run:
-          pip install flake8 flake8-import-order ruff sphinx sphinx_rtd_theme
-          rstcheck[sphinx] doc8
-      - run: pip install .
+      - run: pip install --group dev .
       - run: flake8 .
       - run: ruff check --output-format=github
       - run: doc8 $(git ls-files '*.rst')

pyproject.toml

@@ -22,16 +22,6 @@ dependencies = [
 ]
 dynamic = ["version"]
 
-[project.optional-dependencies]
-dev = [
-    "doc8",
-    "flake8",
-    "flake8-import-order",
-    "rstcheck[sphinx]",
-    "ruff",
-    "sphinx",
-]
-
 [project.scripts]
 yamllint = "yamllint.cli:run"
 
@@ -40,6 +30,17 @@ homepage = "https://github.com/adrienverge/yamllint"
 repository = "https://github.com/adrienverge/yamllint"
 documentation = "https://yamllint.readthedocs.io"
 
+[dependency-groups]
+dev = [
+    "doc8",
+    "flake8",
+    "flake8-import-order",
+    "rstcheck[sphinx]",
+    "ruff",
+    "sphinx",
+    "sphinx_rtd_theme",
+]
+
 [build-system]
 build-backend = "setuptools.build_meta"
 requires = ["setuptools >= 77.0.3"]