CI: add macOS 15

Author: fridtjof

.github/workflows/main.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04, macos-13, macos-14]
+        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04, macos-13, macos-14, macos-15]
 
     steps:
     - uses: actions/checkout@v3