Allow commas as argument separator for extras names in %python_extras_subpkg
    
    This allows e.g.:
    
        %global extras cli,ghostwriter,pytz,dateutil,lark,numpy,pandas,pytest,redis,zoneinfo,django
        %{pyproject_extras_subpkg -n python3-hypothesis %{extras}}
        ...
        %pyproject_buildrequires -x %{extras}
    
    (Note that %pyproject_extras_subpkg is a tiny wrapper around %python_extras_subpkg.)