Update workflow

Can't automatically format code in a PR. Will instead ask contributors
to format their code if the workflow fails.
For code that is pushed, the auto-formatting workflow works, so we keep it.

.github/workflows/format_and_lint_pr.yaml

@@ -0,0 +1,19 @@
+name: Format and lint code
+on:
+  pull_request:
+permissions:
+  contents: write
+jobs:
+  format_and_lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.x'
+      - name: Run Black
+        uses: psf/black@stable
+      - name: Run Flake8
+        run: flake8 .

.github/workflows/format_and_lint.yaml → .github/workflows/format_and_lint_push.yaml

@@ -1,7 +1,6 @@
 name: Format and lint code
 on:
   push:
-  pull_request:
 permissions:
   contents: write
 jobs: