Add runtime type checking in CI tests

This should prevent TYPE_CHECKING misuse.

It might also found issues not found by static type checking,
typically by the current MyPy.

.github/workflows/ci.yaml

@@ -126,6 +126,10 @@ jobs:
         run: |
           python -m pip install pytest-github-actions-annotate-failures
 
+      - name: Install beartype
+        run: |
+          python -m pip install pytest-beartype
+
       - name: Install xarray
         run: |
           python -m pip install --no-deps -e .
@@ -152,6 +156,7 @@ jobs:
           --cov=xarray
           --cov-report=xml
           --junitxml=pytest.xml
+          --beartype-packages=xarray
 
       - name: Upload test results
         if: always()

pyproject.toml

@@ -41,6 +41,7 @@ dev = [
   "mypy",
   "pre-commit",
   "pytest",
+  "pytest-beartype",
   "pytest-cov",
   "pytest-env",
   "pytest-xdist",