Test multiple Python versions.

Author: stefanseefeld

.github/workflows/test-osx.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.8.10]
+        python-version: ['3.8.10', '3.12']
         cxx: [clang++]
         std: [c++11, c++14] # TODO: c++17 is failing !