Python 2.7 is not supported anymore in opencv-python-188.8.131.52 the support was dropped since 184.108.40.206,
The workaround I found was to install opencv-python version 220.127.116.11 (which is the latest supported for Python 2.7, see this for all releases) like this:
pip2 install opencv-python==18.104.22.168
Hope this helps!!
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