graphics/py-opencv: limit python version to 2.x

Python module of OpenCV 2.4.9 is not compatible with Python 3.x

• both on cmake infrastructure level and on module itself level,

so just mark it as Python 2.x only and remove all the python3
shims - they are don't make any difference anyway.
We can patch that hardly to make it work, but it's better to just
update to latest version that have python3 support out of the box.

PR: 204519 (for tracking)