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.

Requested by @vpisarev in #3047.

Place the built Python module library in a dedicated folder inside of
lib/.  This ensures that even if the Python 2 and Python 3 module names
conflict, they will not overwrite one another.

The previous commit fixing references to PYTHON_NUMPY_INCLUDE_PATH
missed a few unset()s.

filterout all CUDA related headers from python module include headers