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/python2/CMakeLists.txt

@@ -4,6 +4,8 @@ endif()
 
 set(the_description "The python2 bindings")
 set(MODULE_NAME python2)
+# Buildbot requires Python 2 to be in root lib dir
+set(MODULE_INSTALL_SUBDIR "")
 
 set(PYTHON_INCLUDE_PATH ${PYTHON2_INCLUDE_PATH})
 set(PYTHON_NUMPY_INCLUDE_DIRS ${PYTHON2_NUMPY_INCLUDE_DIRS})
@@ -17,6 +19,7 @@ set(PYTHON_VERSION_MINOR ${PYTHON2_VERSION_MINOR})
 include(../common.cmake)
 
 unset(MODULE_NAME)
+unset(MODULE_INSTALL_SUBDIR)
 unset(PYTHON_INCLUDE_PATH)
 unset(PYTHON_NUMPY_INCLUDE_DIRS)
 unset(PYTHON_EXECUTABLE)