Fix missing OpenMP support with Python 3.13

Author: pmachapman

.github/workflows/ci.yml

@@ -21,6 +21,9 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
+      - name: Install OpenMP on macOS
+        if: contains(matrix.os, 'macos')
+        run: brew install libomp
       - name: Set up Python ${{ matrix.python-version }}
         id: setup-python
         uses: actions/setup-python@v6