Fixed an issue with a recursion of cv2 in python

[asenyaev]

This PR fixes an issue opencv/opencv-python#689 as @impact27 suggested.

Python docs said (link):

The contents of this dictionary should not be modified; changes may not affect the values of local and free variables used by the interpreter.

Actual behavior

ImportError: ERROR: recursion is detected during loading of "cv2" binary extensions.

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  Patch to opencv_extra has the same branch name.
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