title: sift = cv2.xfeatures2d.SIFT_create() not working even though have contrib installed
body: <p>So I am trying to use:</p>
  
  <pre><code>sift = cv2.xfeatures2d.SIFT_create()
  </code></pre>
  
  <p>and it is coming up with this error:</p>
  
  <pre><code>cv2.error: OpenCV(3.4.3) C:\projects\opencv-python\opencv_contrib\modules\xfeatures2d\src\sift.cpp:1207: error: (-213:The function/feature is not implemented) 
  This algorithm is patented and is excluded in this configuration; Set OPENCV_ENABLE_NONFREE CMake 
  option and rebuild the library in function 'cv::xfeatures2d::SIFT::create'
  </code></pre>
  
  <p>I am using <code>Python 3.5.0</code> and <code>opencv(3.4.3)</code> and I am just using idle. This occured after I tried to install TensorFlow and I have tried looking around and have installed opencv-contrib-python but I am still getting the same error. Thank you in advance and I apologise if I have not included enough info</p>