GHA: upgrade macOS runner to macos-15

Author: wjakob

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ['ubuntu-latest', 'windows-2022', 'macos-13']
+        os: ['ubuntu-latest', 'windows-2022', 'macos-15']
         python: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14', 'pypy3.10-v7.3.19', 'pypy3.11-v7.3.20']
 
     name: "Python ${{ matrix.python }} / ${{ matrix.os }}"