run stubtest in CI

Author: jorenham

.github/workflows/test.yml

@@ -272,3 +272,18 @@ jobs:
       - name: Check no test always skipped
         run: |
           python continuous_integration/check_no_test_skipped.py test_results
+
+  stubtest:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.9"
+
+      - name: Install dependencies
+        run: pip install mypy .
+
+      - name: Run stubtest
+        run: stubtest threadpoolctl