Build for Python 3.13 freethreading only

Other “python_single_target” USE_EXPAND flag values

Use FlagDescription
python_single_target_pypy3Build for PyPy3 only
python_single_target_python3_10Build for Python 3.10 only
python_single_target_python3_11Build for Python 3.11 only
python_single_target_python3_12Build for Python 3.12 only
python_single_target_python3_13Build for Python 3.13 only
python_single_target_python3_13tBuild for Python 3.13 freethreading only

All packages providing a “python_single_target_python3_13t” USE flag (2)