build: Replace flake8 with ruff

Add isort (I) rules to the default set of ruff rules, as a replacement
for flake8-import-order.

.flake8

@@ -1,4 +0,0 @@
-[flake8]
-import-order-style = pep8
-application-import-names = yamllint
-ignore = W503,W504

.github/workflows/ci.yaml

@@ -21,9 +21,9 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
       - run:
-          pip install flake8 flake8-import-order sphinx rstcheck[sphinx] doc8
+          pip install ruff sphinx rstcheck[sphinx] doc8
       - run: pip install .
-      - run: flake8 .
+      - run: ruff .
       - run: doc8 $(git ls-files '*.rst')
       - run: rstcheck --ignore-directives automodule $(git ls-files '*.rst')
       - run: yamllint --strict $(git ls-files '*.yaml' '*.yml')

.ruff.toml

@@ -0,0 +1,5 @@
+[lint]
+extend-select = ["I"]
+
+[lint.isort]
+known-third-party = ["tests"]

CONTRIBUTING.rst

@@ -23,7 +23,7 @@ Pull Request Process
 
    .. code:: bash
 
-    flake8 .
+    ruff .
 
    If you added/modified documentation:
 

pyproject.toml

@@ -26,9 +26,8 @@ dynamic = ["version"]
 [project.optional-dependencies]
 dev = [
     "doc8",
-    "flake8",
-    "flake8-import-order",
     "rstcheck[sphinx]",
+    "ruff",
     "sphinx",
 ]