fix darker version 1.2.3 linting

it looks like the current version of darker doesn't work properly if
isort isn't installed - see https://github.com/akaihola/darker/pull/132/files
so let's just install isort to fix it.

.github/workflows/python-package.yml

@@ -28,7 +28,7 b' jobs:'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install darker
+        pip install darker isort
     - name: Lint with darker
       run: |
         darker -r 60625f241f298b5039cb2debc365db38aa7bb522 --check --diff . || (