Build Python 3 bindings in subdirectory

Build the Python 3 cv2 module in lib/python3/, to avoid potential naming
conflicts with the Python 2 bindings.

The Python 2 bindings are placed directly in lib/, where they are
required for the Buildbot to successfully execute the Python tests.

modules/python/common.cmake

@@ -72,7 +72,7 @@ execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "import distutils.sysconfig; pri
                 OUTPUT_STRIP_TRAILING_WHITESPACE)
 
 set_target_properties(${the_module} PROPERTIES
-                      LIBRARY_OUTPUT_DIRECTORY  "${LIBRARY_OUTPUT_PATH}/${MODULE_NAME}"
+                      LIBRARY_OUTPUT_DIRECTORY  "${LIBRARY_OUTPUT_PATH}/${MODULE_INSTALL_SUBDIR}"
                       PREFIX ""
                       OUTPUT_NAME cv2
                       SUFFIX ${CVPY_SUFFIX})