Add separate pyupgrade workflow

.github/workflows/pyupgrade.yml

@@ -0,0 +1,23 @@
+---
+name: pyupgrade
+
+on: push  # yamllint disable-line rule:truthy
+
+jobs:
+  pyupgrade:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-python@v3
+      - run: pip install pyupgrade
+      - name: Ensure modern Python style using pyupgrade
+        # This script is written in a Linux / macos / windows portable way
+        run: |
+          python -c "
+          import sys
+          from pyupgrade._main import main
+          from glob import glob
+          files = glob('**/*.py', recursive=True)
+          sys.exit(main(files + ['--py37-plus']))
+          "