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

@@ -1,39 +1,39 b''
 # This workflow will install Python dependencies, run tests and lint with a variety of Python versions
 # For more information see: https://help.github.com/actions/language-and-framework-guides/using-python-with-github-actions
 
 name: Python package
 
 on:
   push:
     branches: [ master, 7.x ]
   pull_request:
     branches: [ master, 7.x ]
 
 jobs:
   build:
 
     runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [3.8]
 
     steps:
     - uses: actions/checkout@v2
       with: 
         fetch-depth: 0
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
     - 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 . || (
         echo "Changes need auto-formatting. Run:"
         echo "    darker -r 60625f241f298b5039cb2debc365db38aa7bb522"
         echo "then commit and push changes to fix."
         exit 1
         )