Add <install dir>/python to PYTHONPATH

This is needed to run prefix commands.

setup.sh.in

 export PATH=@CMAKE_INSTALL_FULL_BINDIR@:$PATH
 export LD_LIBRARY_PATH=@CMAKE_INSTALL_FULL_LIBDIR@:$LD_LIBRARY_PATH
+export PYTHONPATH=@CMAKE_INSTALL_PREFIX@/python:$PYTHONPATH