Pass --install-lib to setup.py install.
pypy installs in /usr/site-packages otherwise...

Index: catkin-0.7.6/cmake/templates/python_distutils_install.sh.in
===================================================================
--- catkin-0.7.6.orig/cmake/templates/python_distutils_install.sh.in
+++ catkin-0.7.6/cmake/templates/python_distutils_install.sh.in
@@ -30,4 +30,6 @@ echo_and_run /usr/bin/env \
     build --build-base "@CMAKE_CURRENT_BINARY_DIR@" \
     install \
     $DESTDIR_ARG \
-    @SETUPTOOLS_ARG_EXTRA@ --prefix="@CMAKE_INSTALL_PREFIX@" --install-scripts="${PYTHON_SCRIPTDIR:-@CMAKE_INSTALL_PREFIX@/@CATKIN_GLOBAL_BIN_DESTINATION@}"
+    @SETUPTOOLS_ARG_EXTRA@ --prefix="@CMAKE_INSTALL_PREFIX@" \
+    --install-scripts="${PYTHON_SCRIPTDIR:-@CMAKE_INSTALL_PREFIX@/@CATKIN_GLOBAL_BIN_DESTINATION@}" \
+    --install-lib="@CMAKE_INSTALL_PREFIX@/@PYTHON_INSTALL_DIR@"