Add main_tests os: macos-11 on python-version: 3.12

Author: cclauss

.github/workflows/ci.yml

@@ -24,6 +24,8 @@ jobs:
             python-version: '3.11'
           - os: ubuntu-latest
             python-version: '3.12'
+          - os: macos-11
+            python-version: '3.12'
 
     steps:
       - uses: actions/checkout@v4