ImportError: numpy.core.multiarray failed to import on windows
Numpy 1.8.1 is very out of date - you should upgrade to the latest version (1.14.1 as of writing) and that error will be resolved.
Out of interest, I've seen this question asked before - are you following a guide that is out of date or something?
I am using python 2.7 on windows 10 . I installed numpy-1.8.1-win32-superpack-python2.7 and extracted opencv-3.4.0-vc14_vc15.
I copied cv2.pyd from opencv\build\python\2.7\x86 and pasted to C:\Python27\Lib\site-packages.
I could import numpy without any error. While I run import cv2 it gives an error like 
RuntimeError: module compiled against API version 0xa but this version of numpy is 0x9
Traceback (most recent call last):
  File "", line 1, in 
    import cv2
ImportError: numpy.core.multiarray failed to import.