MAINT: verify importing numpy does not re-enable the GIL in CI

Author: ngoldbaum

.github/workflows/linux.yml

@@ -310,7 +310,16 @@ jobs:
     - name: Install nightly Cython
       run: |
         pip install git+https://github.com/cython/cython
+    # Set PYTHON_GIL=0 to force GIL off during tests and then manually verify
+    # importing numpy does not enable the GIL. When f2py grows the ability to
+    # declare GIL-disabled support we can just run the tests without the
+    # environment variable
     - uses: ./.github/meson_actions
       env:
-        # needed until f2py grows a config flag for GIL-disabled support
         PYTHON_GIL: 0
+    - name: Verify import does not re-enable GIL
+      run: |
+        if [[ $(python -c "import numpy" 2>&1) == "*The global interpreter lock (GIL) has been enabled*" ]]; then
+            echo "Error: Importing NumPy re-enables the GIL in the free-threaded build"
+            exit 1
+        fi