ci: convert existing CI job to run pyupgrade through Darker

.github/workflows/pyupgrade.yml

@@ -13,12 +13,4 @@ jobs:
         uses: astral-sh/setup-uv@v3
       - uses: actions/setup-python@v5
       - name: Ensure modern Python style using pyupgrade
-        # This script is written in a Linux / macos / windows portable way
-        run: |
-          uvx --from pyupgrade python -c "
-          import sys
-          from pyupgrade._main import main
-          from glob import glob
-          files = glob('**/*.py', recursive=True)
-          sys.exit(main(files + ['--py38-plus']))
-          " || ( git diff ; false )
+        run: uvx --from '.[pyupgrade]' darker --formatter=pyupgrade --diff