Add Python 3 support to the Python test.py script.

The print function is used in place of the print statement.

The urlopen function has been moved to urllib.request in Python 3, so
attempt to import it from either location.

TestCase.assert_() has been deprecated in place of
TestCase.assertTrue().

The tests all pass in both Python 2 and 3.

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