Introduce a check CI job for branch protection (#11434)

This patch adds a special check-job that produces a clear failure or
success status based on how the dependent jobs are doing. It is
possible to use it in GitHub's branch protection instead of having to
manually add and remove individual job names via the repo settings.

https://github.com/marketplace/actions/alls-green#why

Author: webknjaz

.github/workflows/ci.yml

@@ -284,3 +284,34 @@ jobs:
           -m integration
           --verbose --numprocesses auto --showlocals
           --durations=5
+
+  check:  # This job does nothing and is only used for the branch protection
+    if: always()
+
+    needs:
+      - determine-changes
+      - docs
+      - packaging
+      - pre-commit
+      - tests-unix
+      - tests-windows
+      - tests-zipapp
+      - tests-importlib-metadata
+      - vendoring
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Decide whether the needed jobs succeeded or failed
+        uses: re-actors/alls-green@release/v1
+        with:
+          allowed-skips: >-
+            ${{
+              (
+                needs.determine-changes.outputs.vendoring != 'true'
+                && github.event_name == 'pull_request'
+              )
+              && 'vendoring'
+              || ''
+            }}
+          jobs: ${{ toJSON(needs) }}