Revert replacement of flake8 with ruff

Start with reverting parts of commit 26be794 in #629 related to config
and CI jobs.

.flake8

@@ -0,0 +1,4 @@
+[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 ruff sphinx rstcheck[sphinx] doc8
+          pip install flake8 flake8-import-order sphinx rstcheck[sphinx] doc8
       - run: pip install .
-      - run: ruff .
+      - run: flake8 .
       - 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

@@ -1,6 +0,0 @@
-[lint]
-extend-select = ["B", "I", "PGH", "TRY", "UP"]
-ignore = ["TRY003"]
-
-[lint.isort]
-known-third-party = ["tests"]

CONTRIBUTING.rst

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

pyproject.toml

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