Add downstream checks (#20)

Author: blink1073

.github/workflows/downstream.yml

@@ -0,0 +1,48 @@
+name: Test downstream projects
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+
+concurrency:
+  group: downstream-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  ipykernel:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+      - uses: jupyterlab/maintainer-tools/.github/actions/downstream-test@v1
+        with:
+          package_name: ipykernel
+
+  ipywidgets:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Base Setup
+        uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+
+      - name: Run Test
+        uses: jupyterlab/maintainer-tools/.github/actions/downstream-test@v1
+        with:
+          package_name: ipywidgets
+          test_command: pytest -vv -raXxs -k \"not deprecation_fa_icons and not tooltip_deprecation and not on_submit_deprecation\" -W default --durations 10 --color=yes
+
+  downstream_check: # This job does nothing and is only used for the branch protection
+    if: always()
+    needs:
+      - ipykernel
+      - ipywidgets
+    runs-on: ubuntu-latest
+    steps:
+      - name: Decide whether the needed jobs succeeded or failed
+        uses: re-actors/alls-green@release/v1
+        with:
+          jobs: ${{ toJSON(needs) }}