Add github worklow for darker

.github/workflows/darker.yml

@@ -0,0 +1,16 @@
+name: Lint
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - uses: actions/setup-python@v2
+      - uses: akaihola/darker@1.4.2
+        with:
+          options: "--check --diff --isort"
+          version: "1.4.2"

newsfragments/2921.misc.rst

@@ -0,0 +1 @@
+Add a pre-commit hook and github action for `Darker <https://github.com/akaihola/darker>`_ to ensure all future changes conform to black and isort.