Failed building wheel for opencv-python-headless
Running setup.py clean for opencv-python-headless
Failed to build opencv-python-headless
Could not build wheels for opencv-python-headless which use PEP 517 and cannot be installed directly

解决办法：

python -m pip install --upgrade pip -i https://pypi.douban.com/simple
pip3 install ddddocr -i https://pypi.douban.com/simple