Add %python3_platform useful for PYTHONPATH on arched builds
    
    This is mostly used when building Sphinx docs on arched build.
    
    Previously:
    
      PLATFORM=$(python3 -c "import sysconfig; print(sysconfig.get_platform())")
      export PYTHONPATH=../build/lib.${PLATFORM}-%{python3_version}
      make man
    
    Or:
    
      PYTHONPATH=`realpath ../build/lib.linux*` make
    
    Or:
    
      PYTHONPATH=$(echo $PWD/build/lib.linux-*) make html
    
    Now:
    
      PYTHONPATH=../build/lib.%{python3_platform}-%{python3_version}