Add "receive" CI workflow

.github/workflows/pytest.yaml

@@ -6,6 +6,10 @@ on:
   # 05:00 UTC = 06:00 CET = 07:00 CEST
   schedule:
   - cron: "0 5 * * *"
+  workflow_run:
+    workflows: [ "Receive pull request" ]
+    types: [ completed ]
+    branches-ignore: [ main ]
 
 # Cancel previous runs that have not completed
 concurrency:
@@ -23,6 +27,10 @@ env:
 
 jobs:
   pytest:
+    if: >
+      github.event_name != 'workflow_run' ||
+      github.event.workflow_run.conclusion == 'success'
+
     strategy:
       matrix:
         os:

.github/workflows/receive.yaml

@@ -0,0 +1,21 @@
+name: Receive pull request
+
+on:
+  pull_request:
+    branches: [ main ]
+
+env:
+  label: "safe to test"
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Ensure first-party branch or valid label
+      if: >
+        github.repository != github.event.pull_request.head.repo.full_name &&
+        ! contains(github.event.pull_request.labels.*.name, env.label)
+      run: |
+        echo "Will not run \`pytest\` workflow for branch in fork without label \`${{ env.label }}\`." >>$GITHUB_STEP_SUMMARY
+        exit 1