ci: Add a Python type-checking job

.github/workflows/ci.yaml

@@ -365,6 +365,19 @@ jobs:
       - run: pip install black==23.11.0
       - run: black --check --diff .
 
+  # https://github.com/python/mypy
+  mypy:
+    runs-on: ubuntu-22.04
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+      # mypy gets upset about \ in f-strings if Python is too old.
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - run: pip install mypy==1.9.0
+      - run: mypy --strict $(find . -name "*.py")
+
 concurrency:
   group: ${{ github.head_ref || github.run_id }}
   cancel-in-progress: true