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}