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
@@ -33,6 +36,11 @@ jobs:
           virtualenvs-in-project: true
           installer-parallel: true
       - name: Install dependencies
+        env:
+          CPPFLAGS: ${{ runner.os == 'macOS' && '-I/opt/homebrew/opt/libomp/include' || '' }}
+          LDFLAGS: ${{ runner.os == 'macOS' && '-L/opt/homebrew/opt/libomp/lib' || '' }}
+          CFLAGS: ${{ runner.os == 'macOS' && '-Xpreprocessor -fopenmp' || '' }}
+          CXXFLAGS: ${{ runner.os == 'macOS' && '-Xpreprocessor -fopenmp' || '' }}
         run: poetry install --no-interaction --no-root
       - name: Install machine
         run: poetry install --no-interaction