update to 2.4.0 (rhbz#2088759)
    
    Replace manual BuildRequires with automatic python build-dependency
    generator.  This also requires replacing %py3_build/%py3_install with
    the more modern %pyproject_wheel/%pyproject_install macros.
    
    Make use of %pyproject_save_files and %pyproject_files in the process,
    to ensure we keep up with any future changes in the python deployment
    process.
    
    Note that %pyproject_save_files includes `%license COPYING` so we don't
    need to list it separately.
    
    Make test dependencies conditional on `--with tests` (which is the
    default).
    
    While here, re-order the BuildRequires and Requires sections.